The Calculus Course

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Lesson 96 of 148 · Module 4: Applications of Integration

Module 0 — Foundations (12/12)

L1. What calculus is and why it was invented

Module 0  2026-08-04  ·  9:47  ·  practice PDF

Practice problems (3)
Problem 1. A car's velocity is given by a curved graph over time. If you want to find the total distance the car traveled between $t=0$ and $t=10$, are you solving the tangent problem or the area problem? Explain why.
Reveal solution
  1. Distance is the accumulation of velocity over time.
  2. On a graph of velocity versus time, the total distance is represented by the area between the curve and the time axis.
  3. Therefore, this is the area problem.

Answer: The area problem.

Problem 2. You have a curve $f(x)$. You calculate the slope between two points $(x_1, y_1)$ and $(x_2, y_2)$. As you move $x_2$ closer and closer to $x_1$, what specific geometric feature of the curve are you trying to find the slope of?
Reveal solution
  1. The slope between two points is a secant line.
  2. As the points merge into one, the line no longer crosses the curve but just touches it.
  3. A line that touches a curve at a single point is called a tangent line.

Answer: The tangent line.

Problem 3. Why can't we use the standard formula $\text{Area} = \text{length} \times \text{width}$ to find the area under the curve $f(x) = x^2$ from $x=0$ to $x=2$?
Reveal solution
  1. The formula $\text{length} \times \text{width}$ only works for rectangles.
  2. A rectangle has a constant height.
  3. The curve $f(x) = x^2$ has a height that changes at every single point along the x axis.
  4. Because the height is not constant, there is no single 'width' or 'length' that describes the whole shape.

Answer: Because the height of the shape is constantly changing, so it is not a rectangle.

L2. Functions as machines; domain, range, graphs

Module 0  2026-08-04  ·  10:01  ·  practice PDF

Practice problems (3)
Problem 1. Given the function $f(x) = x^{2} + 3x - 5$, find the value of $f(-4)$.
Reveal solution
  1. Substitute $-4$ for every $x$ in the expression: $f(-4) = (-4)^{2} + 3(-4) - 5$.
  2. Calculate the square: $(-4)^{2} = 16$.
  3. Calculate the multiplication: $3(-4) = -12$.
  4. Combine the terms: $16 - 12 - 5 = 4 - 5 = -1$.

Answer: -1

Problem 2. Consider the function $g(x) = \frac{10}{x-3}$. What is the domain of this function?
Reveal solution
  1. Identify the operation that could be undefined: division by zero.
  2. Set the denominator equal to zero to find the restricted value: $x - 3 = 0$.
  3. Solve for $x$: $x = 3$.
  4. The function is defined for all real numbers except $x = 3$.

Answer: x \neq 3

Problem 3. A function $h(x)$ is graphed as a straight line segment starting at $(-1, 2)$ and ending at $(5, 10)$. State the domain and range in interval notation.
Reveal solution
  1. Find the domain by looking at the x-coordinates of the endpoints: the leftmost is $-1$ and the rightmost is $5$.
  2. The domain is $[-1, 5]$.
  3. Find the range by looking at the y-coordinates of the endpoints: the lowest is $2$ and the highest is $10$.
  4. The range is $[2, 10]$.

Answer: Domain: [-1, 5], Range: [2, 10]

L3. Linear functions and the meaning of slope

Module 0  2026-08-04  ·  9:42  ·  practice PDF

Practice problems (3)
Problem 1. Find the equation of the line that passes through the points $(-1, 4)$ and $(2, 10)$.
Reveal solution
  1. Calculate the slope $m = \frac{10 - 4}{2 - (-1)} = \frac{6}{3} = 2$.
  2. Substitute $m = 2$ and the point $(2, 10)$ into $y = mx + c$.
  3. $10 = 2(2) + c \implies 10 = 4 + c \implies c = 6$.
  4. Write the final equation.

Answer: $y = 2x + 6$

Problem 2. A function is defined by $f(x) = -\frac{1}{2}x + 3$. Find the average rate of change between $x = 0$ and $x = 4$.
Reveal solution
  1. Find $f(0) = -\frac{1}{2}(0) + 3 = 3$.
  2. Find $f(4) = -\frac{1}{2}(4) + 3 = -2 + 3 = 1$.
  3. Calculate the average rate of change: $\frac{f(4) - f(0)}{4 - 0} = \frac{1 - 3}{4} = \frac{-2}{4} = -\frac{1}{2}$.

Answer: $-\frac{1}{2}$

Problem 3. A line has a slope of $3$ and passes through the point $(4, -2)$. What is the y-intercept of this line?
Reveal solution
  1. Start with the equation $y = mx + c$.
  2. Substitute $m = 3$, $x = 4$, and $y = -2$.
  3. $-2 = 3(4) + c$.
  4. $-2 = 12 + c$.
  5. Subtract $12$ from both sides to find $c = -14$.

Answer: $-14$

L4. Polynomials and power functions

Module 0  2026-08-04  ·  10:03  ·  practice PDF

Practice problems (3)
Problem 1. Find the roots and describe the end behavior of $f(x) = x^2 - 5x + 6$.
Reveal solution
  1. Set the function to zero: $x^2 - 5x + 6 = 0$.
  2. Factorise the quadratic: $(x - 2)(x - 3) = 0$.
  3. Solve for $x$: $x = 2$ and $x = 3$.
  4. Identify the degree as 2 (even) and the leading coefficient as 1 (positive), so both ends point upwards.

Answer: Roots: $x=2, 3$; End behavior: both ends point up.

Problem 2. Find the roots and describe the end behavior of $f(x) = x^3 - 5x^2 + 6x$.
Reveal solution
  1. Factor out the common term $x$: $f(x) = x(x^2 - 5x + 6)$.
  2. Factorise the quadratic part: $f(x) = x(x - 2)(x - 3)$.
  3. Set each factor to zero: $x = 0$, $x = 2$, and $x = 3$.
  4. Identify the degree as 3 (odd) and the leading coefficient as 1 (positive), so the graph goes down to the left and up to the right.

Answer: Roots: $x=0, 2, 3$; End behavior: down to the left, up to the right.

Problem 3. Does the polynomial $f(x) = x^3 + x$ have any roots other than $x = 0$? Explain using factorisation.
Reveal solution
  1. Factor out the common term $x$: $f(x) = x(x^2 + 1)$.
  2. Set the factors to zero: $x = 0$ or $x^2 + 1 = 0$.
  3. Analyze $x^2 + 1 = 0$: this implies $x^2 = -1$.
  4. Since no real number squared is negative, $x^2 + 1$ has no real roots.

Answer: No, the only real root is $x = 0$.

L5. Exponential functions and the number e (informal)

Module 0  2026-08-04  ·  10:52  ·  practice PDF

Practice problems (3)
Problem 1. A population of cells starts with $20$ cells and doubles every hour. Write the function $f(x)$ for the population after $x$ hours and find the population after $5$ hours.
Reveal solution
  1. The starting amount is $20$ and the growth factor is $2$, so the function is $f(x) = 20 \times 2^x$.
  2. Substitute $x = 5$ into the function: $f(5) = 20 \times 2^5$.
  3. Calculate $2^5 = 32$.
  4. Multiply $20 \times 32 = 640$.

Answer: 640

Problem 2. If a value grows continuously at a rate of $100\%$ per period, starting from $1$ unit, what is the approximate value after $3$ periods? Use $e \approx 2.718$.
Reveal solution
  1. Continuous growth at $100\%$ is modeled by the base $e$.
  2. The function is $f(t) = 1 \times e^t$.
  3. Substitute $t = 3$: $f(3) = e^3$.
  4. Calculate $2.718 \times 2.718 \times 2.718 \approx 20.079$.

Answer: 20.079

Problem 3. Compare the values of $g(x) = x^3$ and $h(x) = 3^x$ when $x = 4$. Which one is larger and by how much?
Reveal solution
  1. Evaluate $g(4) = 4^3 = 4 \times 4 \times 4 = 64$.
  2. Evaluate $h(4) = 3^4 = 3 \times 3 \times 3 \times 3 = 81$.
  3. Compare the two: $81 > 64$.
  4. Find the difference: $81 - 64 = 17$.

Answer: h(x) is larger by 17

L6. Logarithms as inverses

Module 0  2026-08-04  ·  10:26  ·  practice PDF

Practice problems (3)
Problem 1. Solve for $x$ in the equation $3^x = 20$. Give your answer in terms of natural logarithms.
Reveal solution
  1. Take the natural logarithm of both sides: $\ln(3^x) = \ln(20)$
  2. Apply the power law to bring $x$ to the front: $x \ln(3) = \ln(20)$
  3. Divide both sides by $\ln(3)$ to isolate $x$: $x = \frac{\ln(20)}{\ln(3)}$

Answer: $x = \frac{\ln(20)}{\ln(3)}$

Problem 2. Simplify the expression $\log_5(125 \cdot 5^2)$ into a single integer.
Reveal solution
  1. Use the product law: $\log_5(125) + \log_5(5^2)$
  2. Evaluate $\log_5(125)$ since $5^3 = 125$, so $\log_5(125) = 3$
  3. Use the power law for the second term: $2 \log_5(5) = 2 \cdot 1 = 2$
  4. Add the results: $3 + 2 = 5$

Answer: 5

Problem 3. Solve for $x$ in the equation $e^{2x} = 7$.
Reveal solution
  1. Take the natural logarithm of both sides: $\ln(e^{2x}) = \ln(7)$
  2. Since $\ln$ and $e$ are inverses, $\ln(e^{2x}) = 2x$
  3. The equation becomes $2x = \ln(7)$
  4. Divide by 2: $x = \frac{\ln(7)}{2}$

Answer: $x = \frac{\ln(7)}{2}$

L7. Trigonometric functions on the unit circle

Module 0  2026-08-04  ·  11:52  ·  practice PDF

Practice problems (3)
Problem 1. Find the exact values of $\sin(\frac{5\pi}{6})$, $\cos(\frac{5\pi}{6})$, and $\tan(\frac{5\pi}{6})$.
Reveal solution
  1. The angle $\frac{5\pi}{6}$ is in the second quadrant because it is between $\frac{\pi}{2}$ and $\pi$.
  2. The reference angle is $\pi - \frac{5\pi}{6} = \frac{\pi}{6}$ (or $30^\circ$).
  3. For $\frac{\pi}{6}$, the coordinates are $(\frac{\sqrt{3}}{2}, \frac{1}{2})$.
  4. In the second quadrant, $x$ is negative and $y$ is positive, so the point is $(-\frac{\sqrt{3}}{2}, \frac{1}{2})$.
  5. Therefore, $\sin(\frac{5\pi}{6}) = \frac{1}{2}$, $\cos(\frac{5\pi}{6}) = -\frac{\sqrt{3}}{2}$, and $\tan(\frac{5\pi}{6}) = \frac{1/2}{-\sqrt{3}/2} = -\frac{1}{\sqrt{3}} = -\frac{\sqrt{3}}{3}$.

Answer: \sin(\frac{5\pi}{6}) = \frac{1}{2}, \cos(\frac{5\pi}{6}) = -\frac{\sqrt{3}}{2}, \tan(\frac{5\pi}{6}) = -\frac{\sqrt{3}}{3}

Problem 2. Convert $225^\circ$ to radians and find the value of $\cos(225^\circ)$.
Reveal solution
  1. To convert to radians, multiply by $\frac{\pi}{180}$: $225 \times \frac{\pi}{180} = \frac{225\pi}{180} = \frac{5\pi}{4}$.
  2. The angle $\frac{5\pi}{4}$ is in the third quadrant.
  3. The reference angle is $\frac{5\pi}{4} - \pi = \frac{\pi}{4}$.
  4. The cosine of $\frac{\pi}{4}$ is $\frac{\sqrt{2}}{2}$.
  5. In the third quadrant, cosine is negative, so $\cos(225^\circ) = -\frac{\sqrt{2}}{2}$.

Answer: \frac{5\pi}{4} \text{ radians}, \cos(225^\circ) = -\frac{\sqrt{2}}{2}

Problem 3. If $\sin(\theta) = \frac{3}{5}$ and $\theta$ is in the second quadrant, find $\cos(\theta)$ and $\tan(\theta)$.
Reveal solution
  1. Use the identity $\sin^2(\theta) + \cos^2(\theta) = 1$.
  2. Substitute the known value: $(\frac{3}{5})^2 + \cos^2(\theta) = 1 \implies \frac{9}{25} + \cos^2(\theta) = 1$.
  3. Solve for $\cos^2(\theta)$: $\cos^2(\theta) = 1 - \frac{9}{25} = \frac{16}{25}$.
  4. Take the square root: $\cos(\theta) = \pm \frac{4}{5}$.
  5. Since $\theta$ is in the second quadrant, $\cos(\theta)$ must be negative, so $\cos(\theta) = -\frac{4}{5}$.
  6. Find tangent: $\tan(\theta) = \frac{\sin(\theta)}{\cos(\theta)} = \frac{3/5}{-4/5} = -\frac{3}{4}$.

Answer: \cos(\theta) = -\frac{4}{5}, \tan(\theta) = -\frac{3}{4}

L8. Inverse trig functions

Module 0  2026-08-04  ·  11:32  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate $\arccos(-\frac{\sqrt{2}}{2})$ in radians.
Reveal solution
  1. We look for an angle $\theta$ such that $\cos(\theta) = -\frac{\sqrt{2}}{2}$ within the range $[0, \pi]$.
  2. The reference angle for $\frac{\sqrt{2}}{2}$ is $\frac{\pi}{4}$.
  3. Since the cosine is negative, the angle must be in the second quadrant.
  4. The angle is $\pi - \frac{\pi}{4} = \frac{3\pi}{4}$.

Answer: \frac{3\pi}{4}

Problem 2. Evaluate $\arctan(-1)$ in radians.
Reveal solution
  1. We look for an angle $\theta$ such that $\tan(\theta) = -1$ within the range $(-\frac{\pi}{2}, \frac{\pi}{2})$.
  2. We know $\tan(\frac{\pi}{4}) = 1$.
  3. Since tangent is an odd function, $\tan(-\frac{\pi}{4}) = -1$.
  4. The angle $-\frac{\pi}{4}$ is within the required range.

Answer: -\frac{\pi}{4}

Problem 3. Find the value of $\arcsin(\sin(\frac{2\pi}{3}))$.
Reveal solution
  1. First, evaluate the inner part: $\sin(\frac{2\pi}{3}) = \frac{\sqrt{3}}{2}$.
  2. Now we need to find $\arcsin(\frac{\sqrt{3}}{2})$.
  3. We look for an angle $\theta$ in the range $[-\frac{\pi}{2}, \frac{\pi}{2}]$ such that $\sin(\theta) = \frac{\sqrt{3}}{2}$.
  4. The angle $\frac{\pi}{3}$ satisfies this and is within the range.

Answer: \frac{\pi}{3}

L9. Composite functions and transformations

Module 0  2026-08-05  ·  11:32  ·  practice PDF

Practice problems (3)
Problem 1. Given $f(x) = x^2$ and $g(x) = 3x - 2$, find the expression for $f(g(x))$ and simplify it.
Reveal solution
  1. Identify the inner function $g(x) = 3x - 2$.
  2. Substitute $g(x)$ into $f(x)$: $f(g(x)) = (3x - 2)^2$.
  3. Expand the square: $(3x)^2 - 2(3x)(2) + 2^2$.
  4. Simplify the terms: $9x^2 - 12x + 4$.

Answer: $f(g(x)) = 9x^2 - 12x + 4$

Problem 2. Let $f(x) = \sin(x)$. Describe the transformations required to obtain the graph of $h(x) = 3 \sin(x - \frac{\pi}{4}) + 1$.
Reveal solution
  1. The multiplier $3$ outside the function indicates a vertical stretch by a factor of $3$.
  2. The term $-\frac{\pi}{4}$ inside the function indicates a horizontal shift to the right by $\frac{\pi}{4}$ units.
  3. The constant $+1$ at the end indicates a vertical shift upwards by $1$ unit.

Answer: Vertical stretch by 3, horizontal shift right by $\frac{\pi}{4}$, and vertical shift up by 1.

Problem 3. Given $f(x) = \frac{1}{x}$ and $g(x) = \sin(x)$, find the domain of $f(g(x))$ for $x$ in the interval $[0, \pi]$.
Reveal solution
  1. The composite function is $f(g(x)) = \frac{1}{\sin(x)}$.
  2. The inner function $g(x) = \sin(x)$ is defined for all real numbers.
  3. The outer function $f(u) = \frac{1}{u}$ is undefined where $u = 0$. Thus, we need $\sin(x) \neq 0$.
  4. In the interval $[0, \pi]$, $\sin(x) = 0$ at $x = 0$ and $x = \pi$.

Answer: $x \neq 0, \pi$

L10. Inequalities, absolute value, interval notation

Module 0  2026-08-06  ·  11:50  ·  practice PDF

Practice problems (3)
Problem 1. Solve the inequality $3x - 7 \ge 14$ and write the answer in interval notation.
Reveal solution
  1. Add $7$ to both sides to get $3x \ge 21$.
  2. Divide both sides by $3$ to get $x \ge 7$.
  3. Since $x$ is greater than or equal to $7$, the interval starts at $7$ and goes to infinity.
  4. Use a square bracket for $7$ because it is included.

Answer: [7, \infty)

Problem 2. Solve the equation $|2x + 5| = 11$.
Reveal solution
  1. Set up two cases: $2x + 5 = 11$ and $2x + 5 = -11$.
  2. For the first case, subtract $5$ to get $2x = 6$, then divide by $2$ to get $x = 3$.
  3. For the second case, subtract $5$ to get $2x = -16$, then divide by $2$ to get $x = -8$.
  4. Both values satisfy the original equation.

Answer: x = 3, x = -8

Problem 3. Solve the inequality $|x - 3| > 2$ and write the answer in interval notation.
Reveal solution
  1. The distance from $x$ to $3$ must be greater than $2$.
  2. This means $x - 3 > 2$ or $x - 3 < -2$.
  3. Solving the first part: $x > 5$, which is $(5, \infty)$.
  4. Solving the second part: $x < 1$, which is $(-\infty, 1)$.
  5. Combine these two disjoint sets using the union symbol.

Answer: (-\infty, 1) \cup (5, \infty)

L11. The idea of 'approaching' — an informal taste of limits

Module 0  2026-08-07  ·  10:37  ·  practice PDF

Practice problems (3)
Problem 1. Find the value that $f(x) = x^2 + 2$ approaches as $x$ approaches $1$.
Reveal solution
  1. Step 1: Plug in values close to $1$ from the left, such as $0.9$ and $0.99$. $f(0.9) = 0.81 + 2 = 2.81$ and $f(0.99) = 0.9801 + 2 = 2.9801$.
  2. Step 2: Plug in values close to $1$ from the right, such as $1.1$ and $1.01$. $f(1.1) = 1.21 + 2 = 3.21$ and $f(1.01) = 1.0201 + 2 = 3.0201$.
  3. Step 3: Observe that both sides are approaching the value $3$.

Answer: 3

Problem 2. Determine the limit of $f(x) = \frac{x^2 - 16}{x - 4}$ as $x$ approaches $4$.
Reveal solution
  1. Step 1: Notice that plugging in $x = 4$ results in $0/0$, which is undefined.
  2. Step 2: Factor the numerator as a difference of squares: $x^2 - 16 = (x - 4)(x + 4)$.
  3. Step 3: Simplify the function: $f(x) = \frac{(x - 4)(x + 4)}{x - 4} = x + 4$ for $x \neq 4$.
  4. Step 4: As $x$ approaches $4$, the expression $x + 4$ approaches $4 + 4 = 8$.

Answer: 8

Problem 3. Consider the function $f(x) = 5$ for $x < 2$ and $f(x) = 10$ for $x \ge 2$. Does the limit exist as $x$ approaches $2$? Explain why or why not.
Reveal solution
  1. Step 1: Approach $x = 2$ from the left. For all $x < 2$, $f(x) = 5$, so the left-hand limit is $5$.
  2. Step 2: Approach $x = 2$ from the right. For all $x \ge 2$, $f(x) = 10$, so the right-hand limit is $10$.
  3. Step 3: Compare the two sides. Since $5 \neq 10$, the function does not approach a single value.

Answer: No, the limit does not exist because the left-hand and right-hand limits are different.

L12. Module 0 review + self-test

Module 0  2026-08-08  ·  11:25  ·  practice PDF

Practice problems (3)
Problem 1. Find the limit: $\lim_{x \to -2} \frac{x^2 + 5x + 6}{x + 2}$
Reveal solution
  1. Factorise the numerator: $x^2 + 5x + 6 = (x + 2)(x + 3)$
  2. Rewrite the limit: $\lim_{x \to -2} \frac{(x + 2)(x + 3)}{x + 2}$
  3. Cancel the common factor $(x + 2)$ for $x \neq -2$: $\lim_{x \to -2} (x + 3)$
  4. Substitute $x = -2$ into the simplified expression: $-2 + 3 = 1$

Answer: 1

Problem 2. Solve the inequality $|3x + 1| > 7$ and write the answer in interval notation.
Reveal solution
  1. Split the absolute value into two cases: $3x + 1 > 7$ or $3x + 1 < -7$
  2. Solve the first case: $3x > 6 \implies x > 2$
  3. Solve the second case: $3x < -8 \implies x < -8/3$
  4. Combine the results into interval notation: $(-\infty, -8/3) \cup (2, \infty)$

Answer: (-\infty, -8/3) \cup (2, \infty)

Problem 3. Let $f(x) = e^x$ and $g(x) = 2x - 5$. Find the expression for $(f \circ g)(x)$ and determine the value of $(f \circ g)(3)$.
Reveal solution
  1. Substitute $g(x)$ into $f(x)$: $(f \circ g)(x) = f(2x - 5) = e^{2x - 5}$
  2. Substitute $x = 3$ into the composite function: $e^{2(3) - 5}$
  3. Simplify the exponent: $e^{6 - 5} = e^1$
  4. The final value is $e$

Answer: e

Module 1 — Limits & Continuity (14/14)

L13. The limit concept: numerical and graphical intuition

Module 1  2026-08-09  ·  10:52  ·  practice PDF

Practice problems (3)
Problem 1. Use a table of values to estimate the limit: $\lim_{x \to 3} \frac{x^2 - 9}{x - 3}$.
Reveal solution
  1. Pick values approaching 3 from the left: $x = 2.9, 2.99, 2.999$.
  2. Calculate $f(2.9) = \frac{8.41 - 9}{2.9 - 3} = \frac{-0.59}{-0.1} = 5.9$.
  3. Calculate $f(2.99) = \frac{8.9401 - 9}{2.99 - 3} = \frac{-0.0599}{-0.01} = 5.99$.
  4. Pick values approaching 3 from the right: $x = 3.1, 3.01, 3.001$.
  5. Calculate $f(3.1) = \frac{9.61 - 9}{3.1 - 3} = \frac{0.61}{0.1} = 6.1$.
  6. Calculate $f(3.01) = \frac{9.0601 - 9}{3.01 - 3} = \frac{0.0601}{0.01} = 6.01$.
  7. Observe that both sides approach 6.

Answer: 6

Problem 2. Given the piecewise function $f(x) = x + 5$ for $x < 1$ and $f(x) = 2x + 4$ for $x \ge 1$, find $\lim_{x \to 1} f(x)$.
Reveal solution
  1. Find the left-hand limit: $\lim_{x \to 1^{-}} (x + 5) = 1 + 5 = 6$.
  2. Find the right-hand limit: $\lim_{x \to 1^{+}} (2x + 4) = 2(1) + 4 = 6$.
  3. Since the left-hand limit equals the right-hand limit, the general limit exists.

Answer: 6

Problem 3. Determine if the limit $\lim_{x \to 0} \frac{1}{x}$ exists. Explain using one-sided limits.
Reveal solution
  1. Approach 0 from the right: as $x \to 0^{+}$, $1/x$ becomes very large, so $\lim_{x \to 0^{+}} \frac{1}{x} = \infty$.
  2. Approach 0 from the left: as $x \to 0^{-}$, $1/x$ becomes very large and negative, so $\lim_{x \to 0^{-}} \frac{1}{x} = -\infty$.
  3. Since the left-hand and right-hand limits are not equal (and are not finite), the limit does not exist.

Answer: Does Not Exist

L14. One-sided limits; when limits fail to exist

Module 1  2026-08-10  ·  9:51  ·  practice PDF

Practice problems (3)
Problem 1. Find the limit $\lim_{x \to -1} f(x)$ for the piecewise function $f(x) = x^2$ if $x < -1$ and $f(x) = -x$ if $x \ge -1$.
Reveal solution
  1. Compute the left-hand limit: $\lim_{x \to -1^{-}} x^2 = (-1)^2 = 1$.
  2. Compute the right-hand limit: $\lim_{x \to -1^{+}} (-x) = -(-1) = 1$.
  3. Since the left-hand limit equals the right-hand limit, the overall limit exists.

Answer: 1

Problem 2. Determine if the limit $\lim_{x \to 2} \frac{1}{(x-2)^2}$ exists. If not, explain why.
Reveal solution
  1. As $x$ approaches $2$ from the right, $(x-2)^2$ is a very small positive number, so $1/(x-2)^2$ goes to $\infty$.
  2. As $x$ approaches $2$ from the left, $(x-2)^2$ is also a very small positive number, so $1/(x-2)^2$ goes to $\infty$.
  3. Because the function is unbounded, it does not approach a finite real number.

Answer: DNE (Unbounded)

Problem 3. Consider the function $f(x) = \frac{|x+3|}{x+3}$. Find $\lim_{x \to -3} f(x)$.
Reveal solution
  1. For $x > -3$, $|x+3| = x+3$, so $f(x) = (x+3)/(x+3) = 1$. The right-hand limit is $1$.
  2. For $x < -3$, $|x+3| = -(x+3)$, so $f(x) = -(x+3)/(x+3) = -1$. The left-hand limit is $-1$.
  3. Since $1 \neq -1$, the limit does not exist.

Answer: DNE (Jump)

L15. Limit laws and evaluating limits algebraically

Module 1  2026-08-11  ·  12:04  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit: $\lim_{x \to 5} \frac{x^2 - 25}{x - 5}$
Reveal solution
  1. Substitute $x = 5$ to check for indeterminate form: $\frac{5^2 - 25}{5 - 5} = \frac{0}{0}$.
  2. Factor the numerator as a difference of squares: $x^2 - 25 = (x - 5)(x + 5)$.
  3. Cancel the common factor $(x - 5)$ from the top and bottom: $\lim_{x \to 5} (x + 5)$.
  4. Use direct substitution: $5 + 5 = 10$.

Answer: 10

Problem 2. Evaluate the limit: $\lim_{x \to 2} (4x^3 - 2x + 7)$
Reveal solution
  1. Recognize that the function is a polynomial, so we can use direct substitution.
  2. Substitute $x = 2$ into the expression: $4(2)^3 - 2(2) + 7$.
  3. Calculate the powers: $4(8) - 4 + 7$.
  4. Perform the multiplication and addition: $32 - 4 + 7 = 35$.

Answer: 35

Problem 3. Evaluate the limit: $\lim_{x \to 1} \frac{x^2 + 2x - 3}{x^2 - 1}$
Reveal solution
  1. Substitute $x = 1$ to check: $\frac{1 + 2 - 3}{1 - 1} = \frac{0}{0}$.
  2. Factor the numerator: $x^2 + 2x - 3 = (x - 1)(x + 3)$.
  3. Factor the denominator: $x^2 - 1 = (x - 1)(x + 1)$.
  4. Cancel the common factor $(x - 1)$: $\lim_{x \to 1} \frac{x + 3}{x + 1}$.
  5. Substitute $x = 1$: $\frac{1 + 3}{1 + 1} = \frac{4}{2} = 2$.

Answer: 2

L16. The squeeze theorem (with sin x over x)

Module 1  2026-08-12  ·  10:34  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit $\lim_{x \to 0} x^2 \sin\left(\frac{5}{x}\right)$ using the Squeeze Theorem.
Reveal solution
  1. Start with the known bound for sine: $-1 \le \sin\left(\frac{5}{x}\right) \le 1$.
  2. Multiply the entire inequality by $x^2$. Since $x^2 \ge 0$ for all $x$, the inequality signs remain the same: $-x^2 \le x^2 \sin\left(\frac{5}{x}\right) \le x^2$.
  3. Evaluate the limit of the lower bound: $\lim_{x \to 0} -x^2 = 0$.
  4. Evaluate the limit of the upper bound: $\lim_{x \to 0} x^2 = 0$.
  5. Since both bounds approach $0$, by the Squeeze Theorem, the middle limit must also be $0$.

Answer: 0

Problem 2. Given that $2x - 1 \le f(x) \le x^2 + 1$ for all $x$, find $\lim_{x \to 1} f(x)$.
Reveal solution
  1. Identify the lower bound function $g(x) = 2x - 1$ and the upper bound function $h(x) = x^2 + 1$.
  2. Calculate the limit of the lower bound as $x$ approaches $1$: $\lim_{x \to 1} (2x - 1) = 2(1) - 1 = 1$.
  3. Calculate the limit of the upper bound as $x$ approaches $1$: $\lim_{x \to 1} (x^2 + 1) = 1^2 + 1 = 2$.
  4. Observe that the limits are $1$ and $2$. Since $1 \neq 2$, the Squeeze Theorem cannot be used to find a specific value for the limit.

Answer: The limit cannot be determined using the Squeeze Theorem because the bounds do not approach the same value.

Problem 3. Use the result $\lim_{x \to 0} \frac{\sin(x)}{x} = 1$ and limit laws to evaluate $\lim_{x \to 0} \frac{\sin(3x)}{x}$.
Reveal solution
  1. We want the expression inside the sine to match the denominator. We have $3x$ inside the sine, but only $x$ in the denominator.
  2. Multiply the expression by $\frac{3}{3}$ to manipulate the denominator: $\lim_{x \to 0} 3 \cdot \frac{\sin(3x)}{3x}$.
  3. Use the constant multiple law to move the $3$ outside the limit: $3 \cdot \lim_{x \to 0} \frac{\sin(3x)}{3x}$.
  4. As $x$ approaches $0$, the term $3x$ also approaches $0$. Therefore, $\lim_{x \to 0} \frac{\sin(3x)}{3x} = 1$ based on the fundamental limit proven today.
  5. The final result is $3 \cdot 1 = 3$.

Answer: 3

L17. Limits at infinity and horizontal asymptotes

Module 1  2026-08-13  ·  11:38  ·  practice PDF

Practice problems (3)
Problem 1. Find the limit as $x$ approaches infinity of the function $f(x) = \frac{6x^2 - 3x + 1}{2x^2 + 5}$.
Reveal solution
  1. Identify the highest power of $x$ in the denominator, which is $x^2$.
  2. Divide every term in the numerator and denominator by $x^2$: $\frac{6 - 3/x + 1/x^2}{2 + 5/x^2}$.
  3. Apply the limit as $x \to \infty$, noting that $3/x$, $1/x^2$, and $5/x^2$ all approach $0$.
  4. The expression simplifies to $6/2 = 3$.

Answer: 3

Problem 2. Determine the horizontal asymptote(s) of the function $g(x) = \frac{5x + 2}{x^2 - 9}$.
Reveal solution
  1. Compare the degrees of the numerator and denominator. The numerator degree is $1$ and the denominator degree is $2$.
  2. Since the degree of the denominator is higher, the limit as $x \to \infty$ is $0$.
  3. Similarly, the limit as $x \to {-\infty}$ is $0$.
  4. The horizontal asymptote is the line $y = 0$.

Answer: y = 0

Problem 3. Evaluate the limit as $x$ approaches infinity of $h(x) = \frac{2x^3 - 4x}{x^3 + 7x^2 + 1}$.
Reveal solution
  1. Divide all terms by the highest power in the denominator, $x^3$.
  2. The expression becomes $\frac{2 - 4/x^2}{1 + 7/x + 1/x^3}$.
  3. As $x \to \infty$, the terms $4/x^2$, $7/x$, and $1/x^3$ all approach $0$.
  4. The limit is $2/1 = 2$.

Answer: 2

L18. Infinite limits and vertical asymptotes

Module 1  2026-08-14  ·  11:11  ·  practice PDF

Practice problems (3)
Problem 1. Find the vertical asymptote and the one-sided limits as $x$ approaches that asymptote for $f(x) = \frac{4}{x + 5}$.
Reveal solution
  1. Set the denominator to zero: $x + 5 = 0$, which gives $x = -5$.
  2. Since the numerator is a non-zero constant, $x = -5$ is a vertical asymptote.
  3. Test the right-hand limit: for $x = -4.9$, $f(-4.9) = \frac{4}{0.1} = 40$ (positive), so $\lim_{x \to -5^+} f(x) = \infty$.
  4. Test the left-hand limit: for $x = -5.1$, $f(-5.1) = \frac{4}{-0.1} = -40$ (negative), so $\lim_{x \to -5^-} f(x) = -\infty$.

Answer: Vertical asymptote at $x = -5$; $\lim_{x \to -5^+} f(x) = \infty$ and $\lim_{x \to -5^-} f(x) = -\infty$.

Problem 2. Determine the vertical asymptote(s) and the behavior of $f(x) = \frac{1}{(x-2)^2}$ as $x$ approaches the asymptote from both sides.
Reveal solution
  1. The denominator is zero when $(x-2)^2 = 0$, which means $x = 2$.
  2. The numerator is $1$, so $x = 2$ is a vertical asymptote.
  3. Test the right-hand limit: for $x = 2.1$, $(2.1-2)^2 = 0.01$, and $1/0.01 = 100$ (positive), so $\lim_{x \to 2^+} f(x) = \infty$.
  4. Test the left-hand limit: for $x = 1.9$, $(1.9-2)^2 = (-0.1)^2 = 0.01$, and $1/0.01 = 100$ (positive), so $\lim_{x \to 2^-} f(x) = \infty$.

Answer: Vertical asymptote at $x = 2$; $\lim_{x \to 2^+} f(x) = \infty$ and $\lim_{x \to 2^-} f(x) = \infty$.

Problem 3. Find all vertical asymptotes for $f(x) = \frac{x-3}{x^2 - 5x + 6}$.
Reveal solution
  1. Factor the denominator: $x^2 - 5x + 6 = (x-2)(x-3)$.
  2. The function is $f(x) = \frac{x-3}{(x-2)(x-3)}$.
  3. The value $x = 3$ makes both numerator and denominator zero, so it is a hole.
  4. The value $x = 2$ makes only the denominator zero, so it is a vertical asymptote.
  5. Simplify the function to $f(x) = \frac{1}{x-2}$ for $x \neq 3$ to confirm the behavior at $x=2$.

Answer: The only vertical asymptote is $x = 2$.

L19. Continuity: definition and types of discontinuity

Module 1  2026-08-15  ·  11:57  ·  practice PDF

Practice problems (3)
Problem 1. Determine if the function $f(x) = \begin{cases} x^2 & x \le 2 \\ 2x & x > 2 \end{cases}$ is continuous at $x = 2$.
Reveal solution
  1. Find the function value: $f(2) = 2^2 = 4$.
  2. Find the left-hand limit: $\lim_{x \to 2^-} x^2 = 4$.
  3. Find the right-hand limit: $\lim_{x \to 2^+} 2x = 2(2) = 4$.
  4. Since $\lim_{x \to 2^-} f(x) = \lim_{x \to 2^+} f(x) = f(2) = 4$, the function is continuous.

Answer: Continuous

Problem 2. Identify the type of discontinuity for $f(x) = \frac{x-3}{x^2-9}$ at $x = 3$.
Reveal solution
  1. Check the function value: $f(3) = \frac{0}{0}$, which is undefined.
  2. Compute the limit: $\lim_{x \to 3} \frac{x-3}{(x-3)(x+3)} = \lim_{x \to 3} \frac{1}{x+3} = \frac{1}{6}$.
  3. Since the limit exists as a finite number but the function is undefined, it is a removable discontinuity.

Answer: Removable Discontinuity

Problem 3. Determine the type of discontinuity for $f(x) = \frac{1}{x-5}$ at $x = 5$.
Reveal solution
  1. Check the function value: $f(5) = \frac{1}{0}$, which is undefined.
  2. Check the right-hand limit: $\lim_{x \to 5^+} \frac{1}{x-5} = \infty$.
  3. Check the left-hand limit: $\lim_{x \to 5^-} \frac{1}{x-5} = -\infty$.
  4. Since the limits are infinite, this is an infinite discontinuity.

Answer: Infinite Discontinuity

L20. The Intermediate Value Theorem

Module 1  2026-08-16  ·  12:32  ·  practice PDF

Practice problems (3)
Problem 1. Show that the function $f(x) = x^3 + 2x - 5$ has at least one root in the interval $[1, 2]$.
Reveal solution
  1. Step 1: Note that $f(x)$ is a polynomial, so it is continuous on the closed interval $[1, 2]$.
  2. Step 2: Evaluate the function at the left endpoint: $f(1) = 1^3 + 2(1) - 5 = 1 + 2 - 5 = -2$.
  3. Step 3: Evaluate the function at the right endpoint: $f(2) = 2^3 + 2(2) - 5 = 8 + 4 - 5 = 7$.
  4. Step 4: Since $f(1) = -2$ and $f(2) = 7$, and $0$ is between $-2$ and $7$, the Intermediate Value Theorem guarantees there is at least one $c$ in $(1, 2)$ such that $f(c) = 0$.

Answer: A root exists because $f(1) = -2$ and $f(2) = 7$ and the function is continuous.

Problem 2. Prove that there exists a value $c$ in the interval $[1, 1.5]$ such that $\tan(c) = 2c$. Use the provided values: $\tan(1) \approx 1.557$ and $\tan(1.5) \approx 14.101$.
Reveal solution
  1. Step 1: Define a new function $g(x) = \tan(x) - 2x$. We want to find where $g(c) = 0$.
  2. Step 2: Check continuity. $\tan(x)$ is continuous on $[1, 1.5]$ because the first vertical asymptote is at $\pi/2 \approx 1.57$, which is outside this interval.
  3. Step 3: Evaluate $g(1) = \tan(1) - 2(1) \approx 1.557 - 2 = -0.443$.
  4. Step 4: Evaluate $g(1.5) = \tan(1.5) - 2(1.5) \approx 14.101 - 3 = 11.101$.
  5. Step 5: Since $g(1) < 0$ and $g(1.5) > 0$, the IVT guarantees a root in $(1, 1.5)$.

Answer: A root exists in $(1, 1.5)$ because $g(1) \approx -0.443$ and $g(1.5) \approx 11.101$ and the function is continuous.

Problem 3. Consider $f(x) = e^x - 3x$. Given that $e \approx 2.718$ and $e^2 \approx 7.389$, show that there are at least two roots in the interval $[0, 2]$.
Reveal solution
  1. Step 1: $f(x)$ is continuous on $[0, 2]$ as it is the difference of an exponential and a linear function.
  2. Step 2: Evaluate at $x=0$: $f(0) = e^0 - 3(0) = 1 - 0 = 1$ (positive).
  3. Step 3: Evaluate at $x=1$: $f(1) = e^1 - 3(1) \approx 2.718 - 3 = -0.282$ (negative).
  4. Step 4: Evaluate at $x=2$: $f(2) = e^2 - 3(2) \approx 7.389 - 6 = 1.389$ (positive).
  5. Step 5: There is a sign change between $x=0$ and $x=1$, so there is a root in $(0, 1)$.
  6. Step 6: There is another sign change between $x=1$ and $x=2$, so there is a second root in $(1, 2)$.

Answer: Roots exist in $(0, 1)$ and $(1, 2)$ because the function values change signs twice.

L21. The formal epsilon-delta definition, gently

Module 1  2026-08-17  ·  12:05  ·  practice PDF

Practice problems (3)
Problem 1. Use the formal $\epsilon-\delta$ definition to prove that $\lim_{x \to 3} (4x - 5) = 7$.
Reveal solution
  1. Start with the output inequality: $|(4x - 5) - 7| < \epsilon$.
  2. Simplify the expression: $|4x - 12| < \epsilon$.
  3. Factor out the constant: $4|x - 3| < \epsilon$.
  4. Isolate the input distance: $|x - 3| < \frac{\epsilon}{4}$.
  5. Choose $\delta = \frac{\epsilon}{4}$. If $0 < |x - 3| < \delta$, then $4|x - 3| < \epsilon$, which implies $|(4x - 5) - 7| < \epsilon$.

Answer: \delta = \frac{\epsilon}{4}

Problem 2. Prove that $\lim_{x \to -1} (2x + 8) = 6$ using the formal definition.
Reveal solution
  1. Set up the inequality: $|(2x + 8) - 6| < \epsilon$.
  2. Simplify: $|2x + 2| < \epsilon$.
  3. Factor: $2|x + 1| < \epsilon$.
  4. Solve for the distance from $x$ to $-1$: $|x - (-1)| < \frac{\epsilon}{2}$.
  5. Choose $\delta = \frac{\epsilon}{2}$.

Answer: \delta = \frac{\epsilon}{2}

Problem 3. Given the function $f(x) = -5x + 2$, find the formula for $\delta$ in terms of $\epsilon$ that proves $\lim_{x \to a} f(x) = L$ for any $a$ and $L$.
Reveal solution
  1. Start with $|(-5x + 2) - L| < \epsilon$.
  2. Since $L = -5a + 2$, substitute this in: $|-5x + 2 - (-5a + 2)| < \epsilon$.
  3. Simplify: $|-5x + 5a| < \epsilon$.
  4. Factor out $-5$: $|-5(x - a)| < \epsilon$.
  5. Use absolute value properties: $5|x - a| < \epsilon$.
  6. Divide by 5: $|x - a| < \frac{\epsilon}{5}$.

Answer: \delta = \frac{\epsilon}{5}

L22. Epsilon-delta worked examples

Module 1  2026-08-18  ·  12:22  ·  practice PDF

Practice problems (3)
Problem 1. Prove using the epsilon-delta definition that $\lim_{x \to 4} (3x + 2) = 14$.
Reveal solution
  1. Start with $|(3x + 2) - 14| < \epsilon$.
  2. Simplify to $|3x - 12| < \epsilon$, which is $3|x - 4| < \epsilon$.
  3. This suggests $|x - 4| < \epsilon/3$.
  4. Choose $\delta = \epsilon/3$.
  5. Verify: if $|x - 4| < \delta$, then $3|x - 4| < 3(\epsilon/3) = \epsilon$, so $|(3x + 2) - 14| < \epsilon$.

Answer: \delta = \frac{\epsilon}{3}

Problem 2. Prove using the epsilon-delta definition that $\lim_{x \to 3} x^2 = 9$.
Reveal solution
  1. Start with $|x^2 - 9| = |x - 3||x + 3| < \epsilon$.
  2. Assume $|x - 3| < 1$, so $2 < x < 4$.
  3. Then $|x + 3| < 4 + 3 = 7$.
  4. We need $|x - 3| \cdot 7 < \epsilon$, so $|x - 3| < \epsilon/7$.
  5. Choose $\delta = \min(1, \epsilon/7)$.

Answer: \delta = \min(1, \frac{\epsilon}{7})

Problem 3. Prove using the epsilon-delta definition that $\lim_{x \to 1} (x^2 + 2x) = 3$.
Reveal solution
  1. Start with $|(x^2 + 2x) - 3| = |(x - 1)(x + 3)| < \epsilon$.
  2. Assume $|x - 1| < 1$, so $0 < x < 2$.
  3. Then $|x + 3| < 2 + 3 = 5$.
  4. We need $|x - 1| \cdot 5 < \epsilon$, so $|x - 1| < \epsilon/5$.
  5. Choose $\delta = \min(1, \epsilon/5)$.

Answer: \delta = \min(1, \frac{\epsilon}{5})

L23. Famous limit: (1 + 1/n)^n approaches e

Module 1  2026-08-19  ·  10:46  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit: $\lim_{n \to \infty} (1 + \frac{3}{n})^n$
Reveal solution
  1. Let $m = \frac{n}{3}$, which implies $n = 3m$.
  2. As $n \to \infty$, $m \to \infty$.
  3. Rewrite the limit as $\lim_{m \to \infty} (1 + \frac{1}{m})^{3m}$.
  4. Apply the power rule for limits: $[\lim_{m \to \infty} (1 + \frac{1}{m})^m]^3$.
  5. Substitute the definition of $e$: $e^3$.

Answer: e^3

Problem 2. Evaluate the limit: $\lim_{n \to \infty} (1 + \frac{1}{2n})^{2n}$
Reveal solution
  1. Let $m = 2n$.
  2. As $n \to \infty$, $m \to \infty$.
  3. Substitute $m$ into the expression: $\lim_{m \to \infty} (1 + \frac{1}{m})^m$.
  4. This is the exact definition of $e$.

Answer: e

Problem 3. Evaluate the limit: $\lim_{n \to \infty} (1 - \frac{2}{n})^n$
Reveal solution
  1. Rewrite the expression as $(1 + \frac{-2}{n})^n$.
  2. Let $m = \frac{n}{-2}$, so $n = -2m$.
  3. As $n \to \infty$, $m \to -\infty$.
  4. The limit becomes $\lim_{m \to -\infty} (1 + \frac{1}{m})^{-2m}$.
  5. This is $[\lim_{m \to -\infty} (1 + \frac{1}{m})^m]^{-2}$.
  6. Substitute $e$: $e^{-2}$ or $\frac{1}{e^2}$.

Answer: e^{-2}

L24. Limits of trigonometric combinations — technique clinic

Module 1  2026-08-20  ·  11:09  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit: $\lim_{x \to 0} \frac{\sin(7x)}{2x}$
Reveal solution
  1. Identify that the argument of sine is $7x$ but the denominator is $2x$.
  2. Multiply the numerator and denominator by $7$ to match the argument: $\frac{7}{2} \cdot \frac{\sin(7x)}{7x}$.
  3. Apply the limit $\lim_{u \to 0} \frac{\sin(u)}{u} = 1$ where $u = 7x$.
  4. The result is $\frac{7}{2} \cdot 1 = 3.5$.

Answer: 3.5

Problem 2. Evaluate the limit: $\lim_{x \to 0} \frac{1 - \cos(4x)}{x}$
Reveal solution
  1. Identify that the argument of cosine is $4x$.
  2. To use the formula $\lim_{u \to 0} \frac{1 - \cos(u)}{u} = 0$, we need $4x$ in the denominator.
  3. Multiply the top and bottom by $4$: $4 \cdot \frac{1 - \cos(4x)}{4x}$.
  4. The limit of the fraction part is $0$, so $4 \cdot 0 = 0$.

Answer: 0

Problem 3. Evaluate the limit: $\lim_{x \to 0} \frac{\sin(3x) + 1 - \cos(x)}{x}$
Reveal solution
  1. Split the limit into two separate fractions: $\lim_{x \to 0} \frac{\sin(3x)}{x} + \lim_{x \to 0} \frac{1 - \cos(x)}{x}$.
  2. For the first part, multiply by $3/3$ to get $3 \cdot \lim_{x \to 0} \frac{\sin(3x)}{3x} = 3 \cdot 1 = 3$.
  3. For the second part, use the known limit $\lim_{x \to 0} \frac{1 - \cos(x)}{x} = 0$.
  4. Add the results: $3 + 0 = 3$.

Answer: 3

L25. Common limit pitfalls and how to avoid them

Module 1  2026-08-21  ·  11:34  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit: $\lim_{x \to -2} \frac{x^2 + 5x + 6}{x + 2}$
Reveal solution
  1. Plugging in $x = -2$ gives $\frac{(-2)^2 + 5(-2) + 6}{-2 + 2} = \frac{4 - 10 + 6}{0} = \frac{0}{0}$, which is an indeterminate form.
  2. Factor the numerator: $x^2 + 5x + 6 = (x + 2)(x + 3)$.
  3. Rewrite the limit: $\lim_{x \to -2} \frac{(x + 2)(x + 3)}{x + 2}$.
  4. Cancel the common factor $(x + 2)$ from the top and bottom: $\lim_{x \to -2} (x + 3)$.
  5. Substitute $x = -2$ into the simplified expression: $-2 + 3 = 1$.

Answer: 1

Problem 2. Evaluate the limit: $\lim_{x \to \infty} \frac{3x^2 - 2x + 1}{5x^2 + 7}$
Reveal solution
  1. Identify the highest power of $x$ in the denominator, which is $x^2$.
  2. Divide every term by $x^2$: $\lim_{x \to \infty} \frac{3 - \frac{2}{x} + \frac{1}{x^2}}{5 + \frac{7}{x^2}}$.
  3. Apply the limit laws: $\frac{2}{x}$, $\frac{1}{x^2}$, and $\frac{7}{x^2}$ all approach $0$ as $x$ approaches infinity.
  4. The expression becomes $\frac{3 - 0 + 0}{5 + 0} = \frac{3}{5}$.

Answer: \frac{3}{5}

Problem 3. Evaluate the limit: $\lim_{x \to 0} \frac{|x|}{x^2 + x}$
Reveal solution
  1. Check the limit from the right ($x > 0$): $\lim_{x \to 0^+} \frac{x}{x(x + 1)} = \lim_{x \to 0^+} \frac{1}{x + 1} = 1$.
  2. Check the limit from the left ($x < 0$): $\lim_{x \to 0^-} \frac{-x}{x(x + 1)} = \lim_{x \to 0^-} \frac{-1}{x + 1} = -1$.
  3. Since the left-hand limit ($-1$) and right-hand limit ($1$) are not equal, the two-sided limit does not exist.

Answer: \text{Does Not Exist}

L26. Module 1 review + self-test

Module 1  2026-08-22  ·  10:26  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit: $\lim_{x \to 4} \frac{x^2 - 16}{x - 4}$
Reveal solution
  1. Substitute $x = 4$ to get $\frac{16-16}{4-4} = \frac{0}{0}$, an indeterminate form.
  2. Factor the numerator as a difference of squares: $x^2 - 16 = (x - 4)(x + 4)$.
  3. Cancel the common factor $(x - 4)$ from the top and bottom.
  4. Evaluate the remaining limit: $\lim_{x \to 4} (x + 4) = 4 + 4 = 8$.

Answer: 8

Problem 2. Evaluate the limit: $\lim_{x \to 0} \frac{\sin(7x)}{2x}$
Reveal solution
  1. Identify the form $\frac{\sin(ax)}{bx}$.
  2. Rewrite the expression to isolate the $\frac{\sin(7x)}{7x}$ form: $\frac{7}{2} \cdot \frac{\sin(7x)}{7x}$.
  3. Apply the special limit $\lim_{u \to 0} \frac{\sin u}{u} = 1$ where $u = 7x$.
  4. The result is $\frac{7}{2} \cdot 1 = 3.5$.

Answer: 7/2

Problem 3. Find the horizontal asymptote of $f(x) = \frac{3x^2 - 5}{2x^2 + x + 1}$ by computing the limit as $x \to \infty$.
Reveal solution
  1. Identify the highest power of $x$ in the denominator, which is $x^2$.
  2. Divide every term by $x^2$: $\frac{3 - 5/x^2}{2 + 1/x + 1/x^2}$.
  3. Take the limit as $x \to \infty$, noting that $5/x^2$, $1/x$, and $1/x^2$ all approach $0$.
  4. The limit is $\frac{3 - 0}{2 + 0 + 0} = \frac{3}{2}$.

Answer: y = 3/2

Module 2 — Differential Calculus (30/30)

L27. The derivative as a limit: slope of the tangent

Module 2  2026-08-23  ·  11:00  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = x^2 + 2x$ at $a = 1$ using the limit definition.
Reveal solution
  1. Set up the limit: $\lim_{h \to 0} \frac{(1+h)^2 + 2(1+h) - (1^2 + 2(1))}{h}$
  2. Expand the numerator: $\lim_{h \to 0} \frac{1 + 2h + h^2 + 2 + 2h - 3}{h}$
  3. Simplify the numerator: $\lim_{h \to 0} \frac{4h + h^2}{h}$
  4. Factor and cancel $h$: $\lim_{h \to 0} (4 + h)$
  5. Evaluate the limit as $h \to 0$: $4 + 0 = 4$

Answer: 4

Problem 2. Find the slope of the tangent line to $f(x) = x^2 - 3x$ at $a = 2$.
Reveal solution
  1. Set up the limit: $\lim_{h \to 0} \frac{((2+h)^2 - 3(2+h)) - (2^2 - 3(2))}{h}$
  2. Expand the terms: $\lim_{h \to 0} \frac{4 + 4h + h^2 - 6 - 3h - (4 - 6)}{h}$
  3. Simplify the numerator: $\lim_{h \to 0} \frac{h^2 + h - 2 - (-2)}{h} = \lim_{h \to 0} \frac{h^2 + h}{h}$
  4. Factor and cancel $h$: $\lim_{h \to 0} (h + 1)$
  5. Evaluate the limit: $0 + 1 = 1$

Answer: 1

Problem 3. Determine the derivative of $f(x) = 3x^2 - 5$ at $a = -1$.
Reveal solution
  1. Set up the limit: $\lim_{h \to 0} \frac{3(-1+h)^2 - 5 - (3(-1)^2 - 5)}{h}$
  2. Expand the squared term: $\lim_{h \to 0} \frac{3(1 - 2h + h^2) - 5 - (3 - 5)}{h}$
  3. Simplify the numerator: $\lim_{h \to 0} \frac{3 - 6h + 3h^2 - 5 - (-2)}{h}$
  4. Combine constants: $\lim_{h \to 0} \frac{-6h + 3h^2}{h}$
  5. Factor and cancel $h$: $\lim_{h \to 0} (-6 + 3h)$
  6. Evaluate the limit: $-6$

Answer: -6

L28. The derivative as instantaneous rate of change

Module 2  2026-08-24  ·  10:41  ·  practice PDF

Practice problems (3)
Problem 1. A particle moves along a line with position $s(t) = 3t^2 - 2t$, where $s$ is in meters and $t$ is in seconds. Find the instantaneous velocity at $t = 1$.
Reveal solution
  1. Set up the limit: $v(1) = \lim_{h \to 0} \frac{s(1+h) - s(1)}{h}$
  2. Calculate $s(1) = 3(1)^2 - 2(1) = 1$
  3. Calculate $s(1+h) = 3(1+h)^2 - 2(1+h) = 3(1 + 2h + h^2) - 2 - 2h = 3 + 6h + 3h^2 - 2 - 2h = 1 + 4h + 3h^2$
  4. Substitute into the limit: $\lim_{h \to 0} \frac{(1 + 4h + 3h^2) - 1}{h} = \lim_{h \to 0} \frac{4h + 3h^2}{h}$
  5. Simplify: $\lim_{h \to 0} (4 + 3h) = 4$

Answer: 4 \text{ m/s}

Problem 2. The cost of producing $x$ units of a product is given by $C(x) = 0.5x^2 + 10x + 100$ dollars. Find the marginal cost (the instantaneous rate of change of cost) when $x = 10$.
Reveal solution
  1. Set up the limit for the derivative at $x=10$: $C'(10) = \lim_{h \to 0} \frac{C(10+h) - C(10)}{h}$
  2. Calculate $C(10) = 0.5(100) + 10(10) + 100 = 50 + 100 + 100 = 250$
  3. Calculate $C(10+h) = 0.5(10+h)^2 + 10(10+h) + 100 = 0.5(100 + 20h + h^2) + 100 + 10h + 100 = 50 + 10h + 0.5h^2 + 200 + 10h = 250 + 20h + 0.5h^2$
  4. Substitute into the limit: $\lim_{h \to 0} \frac{(250 + 20h + 0.5h^2) - 250}{h} = \lim_{h \to 0} \frac{20h + 0.5h^2}{h}$
  5. Simplify: $\lim_{h \to 0} (20 + 0.5h) = 20$

Answer: 20 \text{ dollars per unit}

Problem 3. An object's position is given by $s(t) = 2t^2$. Determine the time $t > 0$ at which the instantaneous velocity is exactly $12 \text{ m/s}$.
Reveal solution
  1. Find the general derivative $s'(t)$: $\lim_{h \to 0} \frac{2(t+h)^2 - 2t^2}{h}$
  2. Expand $2(t+h)^2 = 2(t^2 + 2th + h^2) = 2t^2 + 4th + 2h^2$
  3. Simplify the limit: $\lim_{h \to 0} \frac{4th + 2h^2}{h} = \lim_{h \to 0} (4t + 2h) = 4t$
  4. Set the velocity equal to 12: $4t = 12$
  5. Solve for $t$: $t = 3$

Answer: t = 3 \text{ seconds}

L29. Differentiability vs continuity; corners and cusps

Module 2  2026-08-25  ·  11:44  ·  practice PDF

Practice problems (3)
Problem 1. Determine if the function $f(x) = \begin{cases} x^2 + 1 & x \le 0 \\ x + 1 & x > 0 \end{cases}$ is differentiable at $x = 0$.
Reveal solution
  1. Check continuity: $f(0) = 0^2 + 1 = 1$. The limit from the right is $0 + 1 = 1$. It is continuous.
  2. Find the left-hand derivative: $\lim_{h \to 0^-} \frac{(0+h)^2+1 - (0^2+1)}{h} = \lim_{h \to 0^-} \frac{h^2}{h} = 0$.
  3. Find the right-hand derivative: $\lim_{h \to 0^+} \frac{(0+h)+1 - (0+1)}{h} = \lim_{h \to 0^+} \frac{h}{h} = 1$.
  4. Compare the slopes: $0 \neq 1$.
  5. Since the one-sided derivatives are different, the function is not differentiable at $x = 0$.

Answer: Not differentiable at $x = 0$ (it has a corner).

Problem 2. Is the function $f(x) = |2x - 6|$ differentiable at $x = 3$?
Reveal solution
  1. Check continuity: $f(3) = |2(3) - 6| = 0$. The function is continuous.
  2. For $x > 3$, $f(x) = 2x - 6$. The difference quotient is $\frac{2(3+h)-6 - 0}{h} = \frac{2h}{h} = 2$.
  3. For $x < 3$, $f(x) = -(2x - 6) = -2x + 6$. The difference quotient is $\frac{-2(3+h)+6 - 0}{h} = \frac{-2h}{h} = -2$.
  4. The left-hand slope $-2$ does not equal the right-hand slope $2$.

Answer: Not differentiable at $x = 3$.

Problem 3. Explain why $f(x) = x^{1/5}$ is not differentiable at $x = 0$ despite being continuous.
Reveal solution
  1. Check continuity: $f(0) = 0^{1/5} = 0$. It is continuous.
  2. Set up the difference quotient: $\lim_{h \to 0} \frac{h^{1/5} - 0}{h}$.
  3. Simplify the expression: $\frac{h^{1/5}}{h} = h^{1/5 - 1} = h^{-4/5} = \frac{1}{h^{4/5}}$.
  4. Evaluate the limit: as $h \to 0$, $1/h^{4/5}$ approaches infinity.
  5. Since the slope becomes infinite, there is a vertical tangent.

Answer: It has a vertical tangent at $x = 0$.

L30. Derivative of x to the n — proof and practice

Module 2  2026-08-26  ·  9:44  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = x^7$.
Reveal solution
  1. Identify the exponent $n = 7$.
  2. Apply the power rule: bring the $7$ to the front and subtract $1$ from the exponent.
  3. The result is $7x^{7-1}$.

Answer: f'(x) = 7x^6

Problem 2. Find the derivative of $f(x) = \frac{1}{x^4}$.
Reveal solution
  1. Rewrite the function using a negative exponent: $f(x) = x^{-4}$.
  2. Apply the power rule: bring $-4$ to the front and subtract $1$ from the exponent.
  3. Compute $-4 - 1 = -5$.
  4. The derivative is $-4x^{-5}$, which can be rewritten as $-\frac{4}{x^5}$.

Answer: f'(x) = -\frac{4}{x^5}

Problem 3. Find the derivative of $f(x) = \sqrt[5]{x^2}$.
Reveal solution
  1. Rewrite the radical as a fractional exponent: $f(x) = (x^2)^{1/5} = x^{2/5}$.
  2. Apply the power rule: bring $2/5$ to the front and subtract $1$ from the exponent.
  3. Compute $2/5 - 1 = 2/5 - 5/5 = -3/5$.
  4. The derivative is $\frac{2}{5}x^{-3/5}$.

Answer: f'(x) = \frac{2}{5}x^{-3/5}

L31. Sum, difference, constant-multiple rules

Module 2  2026-08-27  ·  12:00  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = 7x^4 - 2x^3 + 5x - 12$.
Reveal solution
  1. Apply the power rule to $7x^4$ to get $7(4x^3) = 28x^3$.
  2. Apply the power rule to $-2x^3$ to get $-2(3x^2) = -6x^2$.
  3. Apply the power rule to $5x$ to get $5(1) = 5$.
  4. The derivative of the constant $-12$ is $0$.

Answer: f'(x) = 28x^3 - 6x^2 + 5

Problem 2. Find the derivative of $g(x) = \frac{1}{4}x^4 + \frac{2}{3}x^3 - 8$.
Reveal solution
  1. Differentiate $\frac{1}{4}x^4$ as $\frac{1}{4}(4x^3) = x^3$.
  2. Differentiate $\frac{2}{3}x^3$ as $\frac{2}{3}(3x^2) = 2x^2$.
  3. The derivative of $-8$ is $0$.

Answer: g'(x) = x^3 + 2x^2

Problem 3. Find the derivative of $h(x) = (2x + 1)(x - 5)$.
Reveal solution
  1. First, expand the expression: $h(x) = 2x^2 - 10x + x - 5 = 2x^2 - 9x - 5$.
  2. Differentiate $2x^2$ to get $4x$.
  3. Differentiate $-9x$ to get $-9$.
  4. The derivative of $-5$ is $0$.

Answer: h'(x) = 4x - 9

L32. Product rule — proof and intuition

Module 2  2026-08-28  ·  10:44  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = x^3 \ln x$.
Reveal solution
  1. Identify $u = x^3$ and $v = \ln x$.
  2. Compute derivatives: $u' = 3x^2$ and $v' = \frac{1}{x}$.
  3. Apply product rule: $f'(x) = (3x^2)(\ln x) + (x^3)(\frac{1}{x})$.
  4. Simplify the second term: $x^3 \cdot \frac{1}{x} = x^2$.
  5. Combine: $f'(x) = 3x^2 \ln x + x^2$.

Answer: f'(x) = x^2(3 \ln x + 1)

Problem 2. Find the derivative of $g(x) = x^2 \sin x$.
Reveal solution
  1. Identify $u = x^2$ and $v = \sin x$.
  2. Compute derivatives: $u' = 2x$ and $v' = \cos x$.
  3. Apply product rule: $g'(x) = (2x)(\sin x) + (x^2)(\cos x)$.
  4. Factor out $x$: $g'(x) = x(2 \sin x + x \cos x)$.

Answer: g'(x) = 2x \sin x + x^2 \cos x

Problem 3. Find the derivative of $h(x) = (x^2 + 1) e^x$.
Reveal solution
  1. Identify $u = x^2 + 1$ and $v = e^x$.
  2. Compute derivatives: $u' = 2x$ and $v' = e^x$.
  3. Apply product rule: $h'(x) = (2x)(e^x) + (x^2 + 1)(e^x)$.
  4. Factor out $e^x$: $h'(x) = e^x(2x + x^2 + 1)$.
  5. Recognize the quadratic as a perfect square: $x^2 + 2x + 1 = (x+1)^2$.

Answer: h'(x) = (x+1)^2 e^x

L33. Quotient rule

Module 2  2026-08-29  ·  10:55  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = \frac{x+2}{x-3}$.
Reveal solution
  1. Identify $f(x) = x+2$ and $g(x) = x-3$.
  2. Compute derivatives: $f'(x) = 1$ and $g'(x) = 1$.
  3. Apply quotient rule: $\frac{(1)(x-3) - (x+2)(1)}{(x-3)^2}$.
  4. Simplify numerator: $x - 3 - x - 2 = -5$.

Answer: f'(x) = \frac{-5}{(x-3)^2}

Problem 2. Find the derivative of $y = \frac{x^2}{2x+1}$.
Reveal solution
  1. Identify high as $x^2$ (derivative $2x$) and low as $2x+1$ (derivative $2$).
  2. Apply rule: $\frac{(2x)(2x+1) - (x^2)(2)}{(2x+1)^2}$.
  3. Expand numerator: $4x^2 + 2x - 2x^2$.
  4. Simplify: $2x^2 + 2x$.

Answer: y' = \frac{2x^2 + 2x}{(2x+1)^2}

Problem 3. Find the derivative of $g(x) = \frac{e^x}{x^2}$. Decide if the quotient rule or rewriting is more efficient.
Reveal solution
  1. Rewriting as $g(x) = e^x \cdot x^{-2}$ allows the product rule.
  2. Using product rule: $g'(x) = (e^x)(x^{-2}) + (e^x)(-2x^{-3})$.
  3. Factor out $e^x$ and $x^{-3}$: $e^x x^{-3}(x - 2)$.
  4. Convert back to fraction: $\frac{e^x(x-2)}{x^3}$.

Answer: g'(x) = \frac{e^x(x-2)}{x^3}

L34. Derivatives of sin and cos from first principles

Module 2  2026-08-30  ·  10:28  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = 3\sin(x) - 4\cos(x)$.
Reveal solution
  1. Apply the linearity of the derivative to separate the terms: $f'(x) = 3\frac{d}{dx}\sin(x) - 4\frac{d}{dx}\cos(x)$.
  2. Use the known derivatives: $\frac{d}{dx}\sin(x) = \cos(x)$ and $\frac{d}{dx}\cos(x) = -\sin(x)$.
  3. Substitute these into the expression: $f'(x) = 3\cos(x) - 4(-\sin(x))$.
  4. Simplify the signs: $f'(x) = 3\cos(x) + 4\sin(x)$.

Answer: f'(x) = 3\cos(x) + 4\sin(x)

Problem 2. Find the derivative of $g(x) = x^2 \cos(x)$.
Reveal solution
  1. Identify this as a product of $u(x) = x^2$ and $v(x) = \cos(x)$.
  2. Apply the product rule: $g'(x) = u'(x)v(x) + u(x)v'(x)$.
  3. Compute the individual derivatives: $u'(x) = 2x$ and $v'(x) = -\sin(x)$.
  4. Substitute back: $g'(x) = (2x)\cos(x) + (x^2)(-\sin(x))$.
  5. Simplify the expression: $g'(x) = 2x\cos(x) - x^2\sin(x)$.

Answer: g'(x) = 2x\cos(x) - x^2\sin(x)

Problem 3. Find the derivative of $h(x) = \frac{\cos(x)}{x+1}$.
Reveal solution
  1. Identify this as a quotient of $u(x) = \cos(x)$ and $v(x) = x+1$.
  2. Apply the quotient rule: $h'(x) = \frac{u'(x)v(x) - u(x)v'(x)}{[v(x)]^2}$.
  3. Compute the individual derivatives: $u'(x) = -\sin(x)$ and $v'(x) = 1$.
  4. Substitute into the formula: $h'(x) = \frac{(-\sin(x))(x+1) - (\cos(x))(1)}{(x+1)^2}$.
  5. Simplify the numerator: $h'(x) = \frac{-(x+1)\sin(x) - \cos(x)}{(x+1)^2}$.

Answer: h'(x) = \frac{-(x+1)\sin(x) - \cos(x)}{(x+1)^2}

L35. Derivatives of the remaining trig functions

Module 2  2026-08-31  ·  11:38  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = 5\sec(x)$.
Reveal solution
  1. Identify the constant multiple as $5$.
  2. Recall that the derivative of $\sec(x)$ is $\sec(x)\tan(x)$.
  3. Apply the linearity rule: $f'(x) = 5 \cdot \frac{d}{dx}\sec(x)$.

Answer: f'(x) = 5\sec(x)\tan(x)

Problem 2. Find the derivative of $g(x) = \tan(x) + \cos(x)$.
Reveal solution
  1. Differentiate $\tan(x)$ to get $\sec^2(x)$.
  2. Differentiate $\cos(x)$ to get $-\sin(x)$.
  3. Combine the results using the sum rule.

Answer: g'(x) = \sec^2(x) - \sin(x)

Problem 3. Find the derivative of $h(x) = x^2 \cot(x)$.
Reveal solution
  1. Use the product rule with $u = x^2$ and $v = \cot(x)$.
  2. Compute $u' = 2x$ and $v' = -\csc^2(x)$.
  3. Apply the formula $h'(x) = u'v + uv'$.
  4. Substitute the values: $h'(x) = (2x)\cot(x) + (x^2)(-\csc^2(x))$.

Answer: h'(x) = 2x\cot(x) - x^2\csc^2(x)

L36. The chain rule — the most important rule (intuition)

Module 2  2026-09-01  ·  11:59  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $y = \ln(x^2 + 3)$.
Reveal solution
  1. Identify the outer function $f(u) = \ln(u)$ and the inner function $u = x^2 + 3$.
  2. Find the derivative of the outer function: $f'(u) = \frac{1}{u}$.
  3. Find the derivative of the inner function: $u' = 2x$.
  4. Apply the chain rule: $\frac{dy}{dx} = \frac{1}{x^2 + 3} \cdot 2x$.

Answer: \frac{2x}{x^2 + 3}

Problem 2. Find the derivative of $y = \cos(4x^3)$.
Reveal solution
  1. Identify the outer function as cosine and the inner function as $4x^3$.
  2. The derivative of the outer function is $-\sin(u)$, evaluated at $u = 4x^3$.
  3. The derivative of the inner function $4x^3$ is $12x^2$.
  4. Multiply them together: $-\sin(4x^3) \cdot 12x^2$.

Answer: -12x^2\sin(4x^3)

Problem 3. Find the derivative of $y = (5x^2 - 2)^7$.
Reveal solution
  1. Identify the outer function as $u^7$ and the inner function as $u = 5x^2 - 2$.
  2. Differentiate the outer layer: $7u^6$, which is $7(5x^2 - 2)^6$.
  3. Differentiate the inner layer: $\frac{d}{dx}(5x^2 - 2) = 10x$.
  4. Multiply the results: $7(5x^2 - 2)^6 \cdot 10x$.

Answer: 70x(5x^2 - 2)^6

L37. Chain rule — technique clinic

Module 2  2026-09-02  ·  11:47  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $y = x \ln(x^2 + 1)$.
Reveal solution
  1. Identify this as a product of $u = x$ and $v = \ln(x^2 + 1)$.
  2. The derivative of $u$ is $u' = 1$.
  3. The derivative of $v$ requires the chain rule: $v' = \frac{1}{x^2 + 1} \cdot 2x = \frac{2x}{x^2 + 1}$.
  4. Apply the product rule: $y' = u'v + uv' = 1 \cdot \ln(x^2 + 1) + x \cdot \frac{2x}{x^2 + 1}$.
  5. Simplify the expression to $\ln(x^2 + 1) + \frac{2x^2}{x^2 + 1}$.

Answer: \frac{dy}{dx} = \ln(x^2 + 1) + \frac{2x^2}{x^2 + 1}

Problem 2. Find the derivative of $y = \sin^4(3x)$.
Reveal solution
  1. Rewrite the function as $y = (\sin(3x))^4$.
  2. The outermost layer is the power of 4. Its derivative is $4(\sin(3x))^3$.
  3. The next layer is the sine function. Its derivative is $\cos(3x)$.
  4. The innermost layer is $3x$. Its derivative is $3$.
  5. Multiply all layers together: $y' = 4\sin^3(3x) \cdot \cos(3x) \cdot 3$.
  6. Simplify to $12\sin^3(3x)\cos(3x)$.

Answer: \frac{dy}{dx} = 12\sin^3(3x)\cos(3x)

Problem 3. Find the derivative of $y = \frac{e^{2x}}{\cos(x)}$.
Reveal solution
  1. Identify this as a quotient with $u = e^{2x}$ and $v = \cos(x)$.
  2. Find $u'$ using the chain rule: $u' = e^{2x} \cdot 2 = 2e^{2x}$.
  3. Find $v'$: $v' = -\sin(x)$.
  4. Apply the quotient rule: $y' = \frac{u'v - uv'}{v^2} = \frac{(2e^{2x})(\cos(x)) - (e^{2x})(-\sin(x))}{(\cos(x))^2}$.
  5. Factor out $e^{2x}$ in the numerator: $y' = \frac{e^{2x}(2\cos(x) + \sin(x))}{\cos^2(x)}$.

Answer: \frac{dy}{dx} = \frac{e^{2x}(2\cos(x) + \sin(x))}{\cos^2(x)}

L38. Derivative of e to the x and why e is special

Module 2  2026-09-03  ·  11:09  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = 7^x$.
Reveal solution
  1. Identify the base $a = 7$.
  2. Apply the formula $\frac{d}{dx} a^x = a^x \ln a$.
  3. Substitute $7$ for $a$ to get $7^x \ln 7$.

Answer: f'(x) = 7^x \ln 7

Problem 2. Find the derivative of $f(x) = e^{5x^3}$.
Reveal solution
  1. Identify the outer function as $e^u$ and the inner function as $u = 5x^3$.
  2. The derivative of the outer function is $e^{5x^3}$.
  3. The derivative of the inner function is $15x^2$.
  4. Multiply them together using the chain rule: $15x^2 e^{5x^3}$.

Answer: f'(x) = 15x^2 e^{5x^3}

Problem 3. Find the derivative of $f(x) = x e^{2x}$.
Reveal solution
  1. Use the product rule with $u = x$ and $v = e^{2x}$.
  2. Compute $u' = 1$ and $v' = 2e^{2x}$ using the chain rule.
  3. Apply the product rule: $f'(x) = u'v + uv' = (1)(e^{2x}) + (x)(2e^{2x})$.
  4. Factor out $e^{2x}$ to simplify: $e^{2x}(1 + 2x)$.

Answer: f'(x) = e^{2x}(1 + 2x)

L39. Derivative of ln x; logarithmic differentiation

Module 2  2026-09-04  ·  10:56  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = \ln(x^3 + 7x)$.
Reveal solution
  1. Identify the inner function $u = x^3 + 7x$.
  2. Compute the derivative of the inner function: $u' = 3x^2 + 7$.
  3. Apply the formula $\frac{d}{dx} \ln(u) = \frac{u'}{u}$.
  4. Substitute the expressions: $f'(x) = \frac{3x^2 + 7}{x^3 + 7x}$.

Answer: $f'(x) = \frac{3x^2 + 7}{x^3 + 7x}$

Problem 2. Use logarithmic differentiation to find the derivative of $y = x^5 \sin x$.
Reveal solution
  1. Take the natural log of both sides: $\ln y = \ln(x^5 \sin x)$.
  2. Use log laws to expand: $\ln y = 5 \ln x + \ln(\sin x)$.
  3. Differentiate implicitly: $\frac{1}{y} \frac{dy}{dx} = \frac{5}{x} + \frac{\cos x}{\sin x}$.
  4. Simplify the trig term: $\frac{1}{y} \frac{dy}{dx} = \frac{5}{x} + \cot x$.
  5. Multiply by $y$: $\frac{dy}{dx} = (x^5 \sin x) (\frac{5}{x} + \cot x)$.

Answer: $\frac{dy}{dx} = (x^5 \sin x) (\frac{5}{x} + \cot x)$

Problem 3. Find the derivative of $f(x) = x^{\cos x}$ using logarithmic differentiation.
Reveal solution
  1. Set $y = x^{\cos x}$ and take the natural log: $\ln y = \ln(x^{\cos x})$.
  2. Bring the exponent down: $\ln y = (\cos x) \ln x$.
  3. Differentiate using the product rule: $\frac{1}{y} \frac{dy}{dx} = (-\sin x)\ln x + (\cos x)\frac{1}{x}$.
  4. Isolate $\frac{dy}{dx}$ by multiplying by $y$: $\frac{dy}{dx} = x^{\cos x} (\frac{\cos x}{x} - \sin x \ln x)$.

Answer: $f'(x) = x^{\cos x} (\frac{\cos x}{x} - \sin x \ln x)$

L40. Derivatives of inverse trig functions

Module 2  2026-09-05  ·  11:35  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = \arcsin(\sqrt{x})$.
Reveal solution
  1. Identify the outer function as $\arcsin(u)$ and the inner function as $u = \sqrt{x}$.
  2. The derivative of $\arcsin(u)$ is $\frac{1}{\sqrt{1-u^2}}$.
  3. The derivative of $\sqrt{x}$ is $\frac{1}{2\sqrt{x}}$.
  4. Apply the chain rule: $f'(x) = \frac{1}{\sqrt{1-(\sqrt{x})^2}} \cdot \frac{1}{2\sqrt{x}}$.
  5. Simplify the expression: $f'(x) = \frac{1}{2\sqrt{x}\sqrt{1-x}}$.

Answer: f'(x) = \frac{1}{2\sqrt{x-x^2}}

Problem 2. Find the derivative of $g(x) = \arctan(e^{2x})$.
Reveal solution
  1. Identify the outer function as $\arctan(u)$ and the inner function as $u = e^{2x}$.
  2. The derivative of $\arctan(u)$ is $\frac{1}{1+u^2}$.
  3. The derivative of $e^{2x}$ is $2e^{2x}$ using the chain rule.
  4. Apply the chain rule: $g'(x) = \frac{1}{1+(e^{2x})^2} \cdot 2e^{2x}$.
  5. Simplify the exponent: $(e^{2x})^2 = e^{4x}$.

Answer: g'(x) = \frac{2e^{2x}}{1+e^{4x}}

Problem 3. Find the derivative of $h(x) = \ln(\arctan(x))$.
Reveal solution
  1. Identify the outer function as $\ln(u)$ and the inner function as $u = \arctan(x)$.
  2. The derivative of $\ln(u)$ is $\frac{1}{u}$.
  3. The derivative of $\arctan(x)$ is $\frac{1}{1+x^2}$.
  4. Apply the chain rule: $h'(x) = \frac{1}{\arctan(x)} \cdot \frac{1}{1+x^2}$.

Answer: h'(x) = \frac{1}{(1+x^2)\arctan(x)}

L41. Implicit differentiation

Module 2  2026-09-06  ·  10:18  ·  practice PDF

Practice problems (3)
Problem 1. Find $\frac{dy}{dx}$ for the relation $x^2 + 3y^2 = 11$.
Reveal solution
  1. Differentiate both sides with respect to $x$: $2x + 6y \frac{dy}{dx} = 0$.
  2. Subtract $2x$ from both sides: $6y \frac{dy}{dx} = -2x$.
  3. Divide by $6y$: $\frac{dy}{dx} = \frac{-2x}{6y}$.
  4. Simplify the fraction: $\frac{dy}{dx} = -\frac{x}{3y}$.

Answer: \frac{dy}{dx} = -\frac{x}{3y}

Problem 2. Find $\frac{dy}{dx}$ for the relation $x^2 y + y^2 x = 6$.
Reveal solution
  1. Apply the product rule to $x^2 y$: $(2xy + x^2 \frac{dy}{dx})$.
  2. Apply the product rule to $y^2 x$: $(2yx \frac{dy}{dx} + y^2)$.
  3. Combine: $2xy + x^2 \frac{dy}{dx} + 2xy \frac{dy}{dx} + y^2 = 0$.
  4. Group $\frac{dy}{dx}$ terms: $\frac{dy}{dx}(x^2 + 2xy) = -2xy - y^2$.
  5. Isolate $\frac{dy}{dx}$: $\frac{dy}{dx} = \frac{-2xy - y^2}{x^2 + 2xy}$.

Answer: \frac{dy}{dx} = -\frac{2xy + y^2}{x^2 + 2xy}

Problem 3. Find $\frac{dy}{dx}$ for the relation $e^y + x = y$.
Reveal solution
  1. Differentiate both sides: $e^y \frac{dy}{dx} + 1 = \frac{dy}{dx}$.
  2. Move $\frac{dy}{dx}$ terms to one side: $e^y \frac{dy}{dx} - \frac{dy}{dx} = -1$.
  3. Factor out $\frac{dy}{dx}$: $\frac{dy}{dx}(e^y - 1) = -1$.
  4. Isolate $\frac{dy}{dx}$: $\frac{dy}{dx} = \frac{-1}{e^y - 1}$.
  5. Alternatively, multiply top and bottom by $-1$: $\frac{dy}{dx} = \frac{1}{1 - e^y}$.

Answer: \frac{dy}{dx} = \frac{1}{1 - e^y}

L42. Higher-order derivatives; notation tour

Module 2  2026-09-07  ·  10:10  ·  practice PDF

Practice problems (3)
Problem 1. Find the second derivative of $f(x) = 3x^3 - 7x^2 + 5$.
Reveal solution
  1. First, find the first derivative using the power rule: $f'(x) = 9x^2 - 14x$.
  2. Next, differentiate the first derivative: $f''(x) = 18x - 14$.

Answer: f''(x) = 18x - 14

Problem 2. Find the second derivative of $g(x) = \cos(x)$.
Reveal solution
  1. The first derivative of $\cos(x)$ is $-\sin(x)$.
  2. The derivative of $-\sin(x)$ is $-\cos(x)$.

Answer: g''(x) = -\cos(x)

Problem 3. Find the third derivative of $h(x) = e^{5x}$.
Reveal solution
  1. The first derivative is $h'(x) = 5e^{5x}$ using the chain rule.
  2. The second derivative is $h''(x) = 5 \cdot 5e^{5x} = 25e^{5x}$.
  3. The third derivative is $h'''(x) = 25 \cdot 5e^{5x} = 125e^{5x}$.

Answer: h'''(x) = 125e^{5x}

L43. Mixed technique clinic I

Module 2  2026-09-08  ·  11:30  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of $f(x) = x^3 \cos(x^2)$.
Reveal solution
  1. Identify the outer rule as the product rule with $u = x^3$ and $v = \cos(x^2)$.
  2. Compute $u' = 3x^2$.
  3. Compute $v'$ using the chain rule: $v' = -\sin(x^2) \cdot 2x = -2x \sin(x^2)$.
  4. Apply the product rule: $f'(x) = (3x^2)(\cos(x^2)) + (x^3)(-2x \sin(x^2))$.
  5. Simplify the expression: $f'(x) = 3x^2 \cos(x^2) - 2x^4 \sin(x^2)$.

Answer: f'(x) = 3x^2 \cos(x^2) - 2x^4 \sin(x^2)

Problem 2. Find the derivative of $g(x) = \frac{\ln(x)}{x^2}$.
Reveal solution
  1. Identify the outer rule as the quotient rule with $u = \ln(x)$ and $v = x^2$.
  2. Compute $u' = 1/x$ and $v' = 2x$.
  3. Apply the quotient rule: $g'(x) = \frac{(1/x)(x^2) - (\ln x)(2x)}{(x^2)^2}$.
  4. Simplify the numerator: $(1/x)(x^2) = x$, so we have $\frac{x - 2x \ln x}{x^4}$.
  5. Factor out $x$ from the numerator and cancel with the denominator: $g'(x) = \frac{x(1 - 2 \ln x)}{x^4} = \frac{1 - 2 \ln x}{x^3}$.

Answer: g'(x) = \frac{1 - 2 \ln x}{x^3}

Problem 3. Find the derivative of $h(x) = e^{x \sin(x)}$.
Reveal solution
  1. Identify the outer rule as the chain rule for the exponential function $e^u$ where $u = x \sin(x)$.
  2. The derivative is $h'(x) = e^{x \sin(x)} \cdot \frac{d}{dx}(x \sin(x))$.
  3. To find the derivative of $x \sin(x)$, use the product rule: $(1)(\sin x) + (x)(\cos x)$.
  4. Combine the results: $h'(x) = e^{x \sin(x)} (\sin x + x \cos x)$.

Answer: h'(x) = e^{x \sin(x)} (\sin x + x \cos x)

L44. Related rates I — setting up problems

Module 2  2026-09-09  ·  11:59  ·  practice PDF

Practice problems (3)
Problem 1. The side length $s$ of a square is increasing at a rate of $5$ cm/s. How fast is the area $A$ increasing when the side length is $8$ cm?
Reveal solution
  1. The relation is $A = s^2$.
  2. Differentiate with respect to $t$: $\frac{dA}{dt} = 2s \frac{ds}{dt}$.
  3. Substitute $s = 8$ and $\frac{ds}{dt} = 5$.
  4. $\frac{dA}{dt} = 2(8)(5) = 80$.

Answer: 80 \text{ cm}^2/\text{s}

Problem 2. A spherical balloon is being inflated so that its radius $r$ increases at $2$ cm/s. How fast is the volume $V$ increasing when the radius is $3$ cm? (Use $V = \frac{4}{3}\pi r^3$)
Reveal solution
  1. The relation is $V = \frac{4}{3}\pi r^3$.
  2. Differentiate with respect to $t$: $\frac{dV}{dt} = 4\pi r^2 \frac{dr}{dt}$.
  3. Substitute $r = 3$ and $\frac{dr}{dt} = 2$.
  4. $\frac{dV}{dt} = 4\pi(3^2)(2) = 4\pi(9)(2) = 72\pi$.

Answer: 72\pi \text{ cm}^3/\text{s}

Problem 3. A point moves along the x-axis and another along the y-axis. The x-point moves away from the origin at $3$ units/s, and the y-point moves toward the origin at $4$ units/s. How fast is the distance $z$ between them changing when $x = 3$ and $y = 4$?
Reveal solution
  1. The relation is $x^2 + y^2 = z^2$.
  2. Differentiate with respect to $t$: $2x \frac{dx}{dt} + 2y \frac{dy}{dt} = 2z \frac{dz}{dt}$.
  3. Find $z$ when $x=3, y=4$: $z = \sqrt{3^2 + 4^2} = 5$.
  4. Substitute $\frac{dx}{dt} = 3$ and $\frac{dy}{dt} = -4$ (since it moves toward the origin).
  5. $2(3)(3) + 2(4)(-4) = 2(5) \frac{dz}{dt} \implies 18 - 32 = 10 \frac{dz}{dt} \implies -14 = 10 \frac{dz}{dt}$.
  6. $\frac{dz}{dt} = -1.4$.

Answer: -1.4 \text{ units/s}

L45. Related rates II — classic problems

Module 2  2026-09-10  ·  13:18  ·  practice PDF

Practice problems (3)
Problem 1. A circle's radius is increasing at a constant rate of $2\text{ cm/s}$. How fast is the area of the circle increasing when the radius is $5\text{ cm}$?
Reveal solution
  1. The area of a circle is $A = \pi r^2$.
  2. Differentiate with respect to time $t$: $\frac{dA}{dt} = 2\pi r \frac{dr}{dt}$.
  3. Substitute the given values $r = 5$ and $\frac{dr}{dt} = 2$.
  4. $\frac{dA}{dt} = 2\pi(5)(2) = 20\pi$.

Answer: $20\pi\text{ cm}^2\text{/s}$

Problem 2. A $13\text{ ft}$ ladder leans against a wall. The top slides down at $1\text{ ft/s}$. How fast is the bottom sliding away from the wall when the top is $5\text{ ft}$ above the ground?
Reveal solution
  1. Equation: $x^2 + y^2 = 13^2$.
  2. Differentiate: $2x \frac{dx}{dt} + 2y \frac{dy}{dt} = 0$.
  3. Find $x$ when $y=5$: $x^2 + 5^2 = 13^2 \implies x^2 = 144 \implies x = 12$.
  4. Substitute $x=12, y=5, \frac{dy}{dt}=-1$: $2(12)\frac{dx}{dt} + 2(5)(-1) = 0$.
  5. $24\frac{dx}{dt} - 10 = 0 \implies \frac{dx}{dt} = \frac{10}{24} = \frac{5}{12}$.

Answer: $\frac{5}{12}\text{ ft/s}$

Problem 3. A conical tank (vertex down) has a height of $10\text{ m}$ and a top radius of $4\text{ m}$. Water flows in at $2\text{ m}^3\text{/min}$. How fast is the water level rising when the water is $5\text{ m}$ deep?
Reveal solution
  1. Volume $V = \frac{1}{3}\pi r^2 h$. Ratio $\frac{r}{h} = \frac{4}{10} = \frac{2}{5}$, so $r = \frac{2}{5}h$.
  2. Substitute $r$: $V = \frac{1}{3}\pi (\frac{2}{5}h)^2 h = \frac{4}{75}\pi h^3$.
  3. Differentiate: $\frac{dV}{dt} = \frac{4}{25}\pi h^2 \frac{dh}{dt}$.
  4. Substitute $\frac{dV}{dt} = 2$ and $h = 5$: $2 = \frac{4}{25}\pi (5)^2 \frac{dh}{dt}$.
  5. $2 = 4\pi \frac{dh}{dt} \implies \frac{dh}{dt} = \frac{1}{2\pi}$.

Answer: $\frac{1}{2\pi}\text{ m/min}$

L46. Linear approximation and differentials

Module 2  2026-09-11  ·  12:10  ·  practice PDF

Practice problems (3)
Problem 1. Use linear approximation to estimate the value of $\sqrt[3]{28}$.
Reveal solution
  1. Let $f(x) = x^{1/3}$. We choose $a = 27$ because it is a perfect cube near $28$.
  2. Calculate $f(27) = 27^{1/3} = 3$.
  3. Find the derivative: $f'(x) = \frac{1}{3}x^{-2/3} = \frac{1}{3(\sqrt[3]{x})^2}$.
  4. Evaluate the derivative at $a = 27$: $f'(27) = \frac{1}{3(3^2)} = \frac{1}{27}$.
  5. Apply the formula: $L(28) = f(27) + f'(27)(28 - 27) = 3 + \frac{1}{27}(1)$.
  6. Simplify to get $3 + 0.037 = 3.037$.

Answer: 3.037

Problem 2. Use differentials to approximate the change in the volume of a sphere if the radius increases from $5$ cm to $5.1$ cm.
Reveal solution
  1. The volume of a sphere is $V = \frac{4}{3}\pi r^3$.
  2. The differential is $dV = V'(r) dr = 4\pi r^2 dr$.
  3. Substitute the known values: $r = 5$ and $dr = 0.1$.
  4. Calculate $dV = 4\pi(5^2)(0.1) = 4\pi(25)(0.1)$.
  5. Simplify to $100\pi(0.1) = 10\pi$.

Answer: 10\pi \text{ cm}^3 \approx 31.4 \text{ cm}^3

Problem 3. A square has a measured side of $10$ cm. If the measurement has a maximum error of $0.05$ cm, use differentials to estimate the maximum error in the calculated area.
Reveal solution
  1. The area of a square is $A = x^2$.
  2. The differential is $dA = 2x dx$.
  3. Substitute the values: $x = 10$ and $dx = 0.05$.
  4. Calculate $dA = 2(10)(0.05)$.
  5. Simplify to $20(0.05) = 1$.

Answer: 1 \text{ cm}^2

L47. L'Hopital's rule I — zero over zero forms

Module 2  2026-09-12  ·  11:08  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit $\lim_{x \to 0} \frac{x^2}{\sin(x)}$.
Reveal solution
  1. Check the form: as $x \to 0$, $x^2 \to 0$ and $\sin(x) \to 0$. This is a $0/0$ form.
  2. Apply L'Hopital's rule: differentiate the numerator to get $2x$ and the denominator to get $\cos(x)$.
  3. Evaluate the new limit: $\lim_{x \to 0} \frac{2x}{\cos(x)}$.
  4. Substitute $x = 0$: $\frac{2(0)}{\cos(0)} = \frac{0}{1} = 0$.

Answer: 0

Problem 2. Evaluate the limit $\lim_{x \to 0} \frac{e^x - 1 - x}{x^2}$.
Reveal solution
  1. Check the form: $e^0 - 1 - 0 = 0$ and $0^2 = 0$. This is $0/0$.
  2. Apply L'Hopital's rule: the derivative of $e^x - 1 - x$ is $e^x - 1$, and the derivative of $x^2$ is $2x$.
  3. Check the new form: $e^0 - 1 = 0$ and $2(0) = 0$. This is still $0/0$.
  4. Apply L'Hopital's rule again: the derivative of $e^x - 1$ is $e^x$, and the derivative of $2x$ is $2$.
  5. Evaluate the limit: $\lim_{x \to 0} \frac{e^x}{2} = \frac{e^0}{2} = \frac{1}{2}$.

Answer: 1/2

Problem 3. Evaluate the limit $\lim_{x \to 0} \frac{\tan(x) - x}{x^3}$.
Reveal solution
  1. Check the form: $\tan(0) - 0 = 0$ and $0^3 = 0$. This is $0/0$.
  2. Apply L'Hopital's rule: the derivative of $\tan(x) - x$ is $\sec^2(x) - 1$, and the derivative of $x^3$ is $3x^2$.
  3. Note that $\sec^2(x) - 1 = \tan^2(x)$. The limit is $\lim_{x \to 0} \frac{\tan^2(x)}{3x^2}$.
  4. This is still $0/0$. Apply L'Hopital's rule again: the derivative of $\tan^2(x)$ is $2\tan(x)\sec^2(x)$, and the derivative of $3x^2$ is $6x$.
  5. The limit is $\lim_{x \to 0} \frac{2\tan(x)\sec^2(x)}{6x} = \lim_{x \to 0} \frac{\tan(x)}{3x} \cdot \sec^2(x)$.
  6. Apply L'Hopital's rule one last time to $\frac{\tan(x)}{3x}$: the derivative of $\tan(x)$ is $\sec^2(x)$ and the derivative of $3x$ is $3$.
  7. The limit becomes $\frac{\sec^2(0)}{3} \cdot \sec^2(0) = \frac{1}{3} \cdot 1 = \frac{1}{3}$.

Answer: 1/3

L48. L'Hopital's rule II — infinity forms and disguises

Module 2  2026-09-13  ·  11:45  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the limit: $\lim_{x \to 0^+} x^2 \ln x$.
Reveal solution
  1. Identify the form: as $x \to 0^+$, $x^2 \to 0$ and $\ln x \to -\infty$. This is a $0 \cdot \infty$ form.
  2. Rewrite as a quotient: $\lim_{x \to 0^+} \frac{\ln x}{1/x^2}$. This is now $-\infty / \infty$.
  3. Apply L'Hopital's rule: the derivative of $\ln x$ is $1/x$ and the derivative of $1/x^2$ is $-2/x^3$.
  4. Simplify the expression: $\frac{1/x}{-2/x^3} = \frac{1}{x} \cdot \frac{x^3}{-2} = -\frac{x^2}{2}$.
  5. Evaluate the limit: as $x \to 0^+$, $-x^2/2 \to 0$.

Answer: 0

Problem 2. Evaluate the limit: $\lim_{x \to \infty} (\sqrt{x^2 + x} - x)$.
Reveal solution
  1. Identify the form: as $x \to \infty$, both terms go to infinity, so this is $\infty - \infty$.
  2. Rationalize the expression by multiplying by the conjugate: $\frac{(\sqrt{x^2+x}-x)(\sqrt{x^2+x}+x)}{\sqrt{x^2+x}+x}$.
  3. Simplify the numerator: $(\sqrt{x^2+x})^2 - x^2 = x^2 + x - x^2 = x$.
  4. The limit is now $\lim_{x \to \infty} \frac{x}{\sqrt{x^2+x}+x}$, which is $\infty / \infty$.
  5. Apply L'Hopital's rule: the derivative of the top is $1$. The derivative of the bottom is $\frac{2x+1}{2\sqrt{x^2+x}} + 1$.
  6. As $x \to \infty$, the fraction $\frac{2x+1}{2\sqrt{x^2+x}}$ approaches $1$.
  7. The limit becomes $1 / (1 + 1) = 1/2$.

Answer: 1/2

Problem 3. Evaluate the limit: $\lim_{x \to 0^+} (1+x)^{1/x}$.
Reveal solution
  1. Identify the form: as $x \to 0^+$, the base $(1+x) \to 1$ and the exponent $1/x \to \infty$. This is a $1^\infty$ form.
  2. Set $y = (1+x)^{1/x}$ and take the natural log: $\ln y = \ln((1+x)^{1/x}) = \frac{1}{x} \ln(1+x)$.
  3. Rewrite as a quotient: $\lim_{x \to 0^+} \frac{\ln(1+x)}{x}$. This is a $0/0$ form.
  4. Apply L'Hopital's rule: the derivative of $\ln(1+x)$ is $1/(1+x)$ and the derivative of $x$ is $1$.
  5. Evaluate the limit: $\lim_{x \to 0^+} \frac{1/(1+x)}{1} = 1$.
  6. Since $\ln y \to 1$, then $y \to e^1 = e$.

Answer: e

L49. Rolle's theorem and the Mean Value Theorem

Module 2  2026-09-14  ·  11:15  ·  practice PDF

Practice problems (3)
Problem 1. Find all values of $c$ that satisfy the Mean Value Theorem for $f(x) = x^3 - x$ on the interval $[0, 2]$.
Reveal solution
  1. Calculate the average slope: $f(0) = 0$ and $f(2) = 2^3 - 2 = 6$. The average slope is $(6 - 0) / (2 - 0) = 3$.
  2. Find the derivative: $f'(x) = 3x^2 - 1$.
  3. Set the derivative equal to the average slope: $3c^2 - 1 = 3$.
  4. Solve for $c$: $3c^2 = 4$, so $c^2 = 4/3$, which means $c = \pm 2/\sqrt{3}$.
  5. Check which value is in the interval $[0, 2]$. Only $c = 2/\sqrt{3}$ is valid.

Answer: $c = \frac{2}{\sqrt{3}}$

Problem 2. Does Rolle's theorem apply to $f(x) = x^{2/3}$ on the interval $[-1, 1]$? If so, find $c$. If not, explain why.
Reveal solution
  1. Check continuity: $f(x)$ is continuous on $[-1, 1]$.
  2. Check endpoints: $f(-1) = (-1)^{2/3} = 1$ and $f(1) = 1^{2/3} = 1$. Endpoints are equal.
  3. Check differentiability: $f'(x) = \frac{2}{3}x^{-1/3} = \frac{2}{3\sqrt[3]{x}}$.
  4. The derivative is undefined at $x = 0$, which is inside the open interval $(-1, 1)$.
  5. Since the function is not differentiable on the open interval, Rolle's theorem does not apply.

Answer: No, because $f(x)$ is not differentiable at $x = 0$.

Problem 3. For $f(x) = \ln(x)$ on the interval $[1, e]$, find the value of $c$ guaranteed by the Mean Value Theorem.
Reveal solution
  1. Calculate the average slope: $f(1) = \ln(1) = 0$ and $f(e) = \ln(e) = 1$. The average slope is $(1 - 0) / (e - 1) = 1 / (e - 1)$.
  2. Find the derivative: $f'(x) = 1/x$.
  3. Set the derivative equal to the average slope: $1/c = 1 / (e - 1)$.
  4. Solve for $c$: $c = e - 1$.
  5. Check if $c$ is in the interval $[1, e]$. Since $e \approx 2.718$, $c \approx 1.718$, which is between $1$ and $2.718$.

Answer: $c = e - 1$

L50. What the MVT really says

Module 2  2026-09-15  ·  9:55  ·  practice PDF

Practice problems (3)
Problem 1. Find the intervals where the function $f(x) = 2x^3 - 3x^2 - 12x$ is increasing and where it is decreasing.
Reveal solution
  1. Find the derivative: $f'(x) = 6x^2 - 6x - 12$.
  2. Factor the derivative: $f'(x) = 6(x^2 - x - 2) = 6(x - 2)(x + 1)$.
  3. Identify critical points where $f'(x) = 0$: $x = 2$ and $x = -1$.
  4. Test intervals: For $x < -1$, $f'(-2) = 6(-4)(-1) = 24 > 0$ (Increasing). For $-1 < x < 2$, $f'(0) = -12 < 0$ (Decreasing). For $x > 2$, $f'(3) = 6(1)(4) = 24 > 0$ (Increasing).

Answer: Increasing on $(-\infty, -1) \cup (2, \infty)$, decreasing on $(-1, 2)$

Problem 2. Prove that the function $f(x) = e^x + x$ is strictly increasing for all real numbers $x$.
Reveal solution
  1. Find the derivative: $f'(x) = e^x + 1$.
  2. Analyze the range of the exponential function: $e^x$ is always greater than $0$ for all $x$.
  3. Therefore, $f'(x) = e^x + 1$ must always be greater than $1$, which means $f'(x) > 0$ for all $x$.
  4. By the Mean Value Theorem, since the derivative is always positive, the function is strictly increasing.

Answer: Since $f'(x) = e^x + 1 > 0$ for all $x$, the function is strictly increasing.

Problem 3. A function $f$ is differentiable on $(0, \infty)$. If $f'(x) = \frac{x-3}{x^2}$, find the interval where the function is decreasing.
Reveal solution
  1. Set the derivative to be less than zero: $\frac{x-3}{x^2} < 0$.
  2. Since $x^2$ is always positive for $x > 0$, the sign of the fraction depends only on the numerator.
  3. Solve $x - 3 < 0$, which gives $x < 3$.
  4. Combine this with the domain $x > 0$ to get the interval $(0, 3)$.

Answer: The function is decreasing on the interval $(0, 3)$

L51. Increasing/decreasing functions; first derivative test

Module 2  2026-09-16  ·  10:03  ·  practice PDF

Practice problems (3)
Problem 1. Find the local maximum and minimum values of $f(x) = x^3 - 3x^2 - 9x + 5$.
Reveal solution
  1. Find the derivative: $f'(x) = 3x^2 - 6x - 9$.
  2. Set $f'(x) = 0$: $3(x^2 - 2x - 3) = 0$, which factors to $3(x-3)(x+1) = 0$. Critical points are $x = -1$ and $x = 3$.
  3. Test intervals: For $x < -1$, $f'(-2) = 15 > 0$ (increasing). For $-1 < x < 3$, $f'(0) = -9 < 0$ (decreasing). For $x > 3$, $f'(4) = 15 > 0$ (increasing).
  4. Classify: At $x = -1$, sign changes $(+) \to (-)$, so it is a local max. At $x = 3$, sign changes $(-) \to (+)$, so it is a local min.
  5. Calculate values: $f(-1) = (-1)^3 - 3(-1)^2 - 9(-1) + 5 = 10$. $f(3) = (3)^3 - 3(3)^2 - 9(3) + 5 = -22$.

Answer: Local maximum of $10$ at $x = -1$; local minimum of $-22$ at $x = 3$.

Problem 2. Determine the intervals of increase and decrease for $f(x) = x^4 - 4x^2$.
Reveal solution
  1. Find the derivative: $f'(x) = 4x^3 - 8x$.
  2. Set $f'(x) = 0$: $4x(x^2 - 2) = 0$. Critical points are $x = 0, x = \sqrt{2}, x = -\sqrt{2}$.
  3. Test intervals: $x < -\sqrt{2}$ (e.g., $-2$): $f'(-2) = -32 + 16 = -16 < 0$ (decreasing).
  4. Interval $-\sqrt{2} < x < 0$ (e.g., $-1$): $f'(-1) = -4 + 8 = 4 > 0$ (increasing).
  5. Interval $0 < x < \sqrt{2}$ (e.g., $1$): $f'(1) = 4 - 8 = -4 < 0$ (decreasing).
  6. Interval $x > \sqrt{2}$ (e.g., $2$): $f'(2) = 32 - 16 = 16 > 0$ (increasing).

Answer: Increasing on $(-\sqrt{2}, 0) \cup (\sqrt{2}, \infty)$; decreasing on $(-\infty, -\sqrt{2}) \cup (0, \sqrt{2})$.

Problem 3. Find the critical points of $f(x) = x^{2/3}$ and use the first derivative test to determine if they are local extrema.
Reveal solution
  1. Find the derivative: $f'(x) = \frac{2}{3}x^{-1/3} = \frac{2}{3\sqrt[3]{x}}$.
  2. Identify critical points: $f'(x)$ is never zero, but it is undefined at $x = 0$. Since $x = 0$ is in the domain of $f$, it is a critical point.
  3. Test intervals: For $x < 0$, $\sqrt[3]{x}$ is negative, so $f'(x) < 0$ (decreasing).
  4. For $x > 0$, $\sqrt[3]{x}$ is positive, so $f'(x) > 0$ (increasing).
  5. Classify: The sign changes from negative to positive at $x = 0$, so it is a local minimum.

Answer: Critical point at $x = 0$, which is a local minimum.

L52. Concavity, inflection points, second derivative test

Module 2  2026-09-17  ·  10:29  ·  practice PDF

Practice problems (3)
Problem 1. Find the intervals of concavity and the x-coordinates of any inflection points for $f(x) = x^4 - 6x^2 + 5$.
Reveal solution
  1. Find the first derivative: $f'(x) = 4x^3 - 12x$.
  2. Find the second derivative: $f''(x) = 12x^2 - 12$.
  3. Set $f''(x) = 0$ to find potential inflection points: $12(x^2 - 1) = 0$, so $x = 1$ and $x = -1$.
  4. Test intervals: for $x < -1$, $f''(-2) = 36 > 0$ (Up); for $-1 < x < 1$, $f''(0) = -12 < 0$ (Down); for $x > 1$, $f''(2) = 36 > 0$ (Up).
  5. Since the sign changes at both $x = -1$ and $x = 1$, both are inflection points.

Answer: Concave up on $(-\infty, -1) \cup (1, \infty)$, concave down on $(-1, 1)$, inflection points at $x = -1, 1$.

Problem 2. Use the second derivative test to classify the critical points of $f(x) = x e^x$.
Reveal solution
  1. Find the first derivative using the product rule: $f'(x) = 1 \cdot e^x + x \cdot e^x = e^x(1 + x)$.
  2. Set $f'(x) = 0$ to find critical points: $e^x(1 + x) = 0$ implies $x = -1$.
  3. Find the second derivative: $f''(x) = e^x(1 + x) + e^x(1) = e^x(x + 2)$.
  4. Evaluate at the critical point: $f''(-1) = e^{-1}(-1 + 2) = e^{-1} \approx 0.368$.
  5. Since $f''(-1) > 0$, the function is concave up at $x = -1$.

Answer: Local minimum at $x = -1$.

Problem 3. Determine if $f(x) = x^5$ has a local maximum, local minimum, or inflection point at $x = 0$.
Reveal solution
  1. Find the first derivative: $f'(x) = 5x^4$. Setting $f'(0) = 0$ confirms $x = 0$ is a critical point.
  2. Find the second derivative: $f''(x) = 20x^3$.
  3. Evaluate at $x = 0$: $f''(0) = 0$. The second derivative test is inconclusive.
  4. Use the first derivative test: for $x < 0$, $f'(x) = 5x^4 > 0$; for $x > 0$, $f'(x) = 5x^4 > 0$.
  5. Since the first derivative does not change sign, it is not a local extremum. Check concavity: for $x < 0$, $f''(x) = 20x^3 < 0$ (Down); for $x > 0$, $f''(x) = 20x^3 > 0$ (Up).
  6. Since concavity changes, it is an inflection point.

Answer: Inflection point at $x = 0$.

L53. Curve sketching masterclass I

Module 2  2026-09-18  ·  12:16  ·  practice PDF

Practice problems (3)
Problem 1. Perform a full curve sketching analysis for $f(x) = \frac{x+1}{x-2}$. Find the domain, asymptotes, first and second derivatives, and sketch the graph.
Reveal solution
  1. Domain is $x \neq 2$. Vertical asymptote at $x = 2$.
  2. Limit as $x \to \infty$ is $1$, so horizontal asymptote is $y = 1$.
  3. Using quotient rule, $f'(x) = \frac{1(x-2) - 1(x+1)}{(x-2)^2} = \frac{-3}{(x-2)^2}$. Since $f'(x) < 0$ everywhere, the function is always decreasing.
  4. Using quotient rule on $f'(x)$, $f''(x) = \frac{6}{(x-2)^3}$. Concave down for $x < 2$ and concave up for $x > 2$.
  5. Intercepts: $y$-intercept at $f(0) = -1/2$, $x$-intercept at $f(x) = 0 \implies x = -1$.

Answer: A hyperbola with asymptotes $x=2, y=1$, always decreasing, concave down on $(-\infty, 2)$ and concave up on $(2, \infty)$.

Problem 2. Analyze and sketch $f(x) = \frac{1}{x^2 + 1}$. Identify all extrema and inflection points.
Reveal solution
  1. Domain is all real numbers. No vertical asymptotes. Horizontal asymptote at $y = 0$.
  2. First derivative $f'(x) = \frac{-2x}{(x^2+1)^2}$. Critical point at $x = 0$. $f'(x) > 0$ for $x < 0$ and $f'(x) < 0$ for $x > 0$. Local max at $(0, 1)$.
  3. Second derivative $f''(x) = \frac{2(3x^2-1)}{(x^2+1)^3}$. Inflection points where $3x^2 = 1$, so $x = \pm 1/\sqrt{3}$.
  4. Concave up for $|x| > 1/\sqrt{3}$ and concave down for $|x| < 1/\sqrt{3}$.

Answer: A bell-shaped curve with a maximum at $(0, 1)$, horizontal asymptote $y=0$, and inflection points at $x = \pm 1/\sqrt{3}$.

Problem 3. Sketch the function $f(x) = \frac{x^2 - 1}{x^2 - 4}$. Find the vertical asymptotes, the horizontal asymptote, and the local extrema.
Reveal solution
  1. Domain is $x \neq \pm 2$. Vertical asymptotes at $x = -2$ and $x = 2$.
  2. Limit as $x \to \infty$ is $1$, so horizontal asymptote is $y = 1$.
  3. First derivative $f'(x) = \frac{2x(x^2-4) - 2x(x^2-1)}{(x^2-4)^2} = \frac{-6x}{(x^2-4)^2}$.
  4. Critical point at $x = 0$. $f'(x) > 0$ for $x < 0$ and $f'(x) < 0$ for $x > 0$. Local max at $(0, 1/4)$.
  5. Intercepts: $y$-intercept at $(0, 1/4)$, $x$-intercepts at $x = \pm 1$.

Answer: A graph with vertical asymptotes $x=\pm 2$, horizontal asymptote $y=1$, and a local maximum at $(0, 1/4)$.

L54. Curve sketching masterclass II

Module 2  2026-09-19  ·  11:40  ·  practice PDF

Practice problems (3)
Problem 1. Sketch the function $f(x) = x^2 e^{-x}$ for $x \ge 0$. Find all critical points and inflection points.
Reveal solution
  1. Find $f'(x) = 2x e^{-x} - x^2 e^{-x} = x e^{-x}(2-x)$.
  2. Set $f'(x) = 0$ to find critical points at $x=0$ and $x=2$.
  3. Find $f''(x) = (2-2x)e^{-x} - (2x-x^2)e^{-x} = e^{-x}(x^2-4x+2)$.
  4. Solve $x^2-4x+2=0$ using the quadratic formula to find inflection points at $x = 2 \pm \sqrt{2}$.
  5. Analyze signs: $f$ increases on $(0,2)$ and decreases on $(2, \infty)$.
  6. Plot the local maximum at $(2, 4e^{-2})$ and the inflection points.

Answer: Local max at $x=2$, inflection points at $x=2-\sqrt{2}$ and $x=2+\sqrt{2}$, horizontal asymptote $y=0$ as $x \to \infty$.

Problem 2. Sketch $f(x) = e^x \cos(x)$ on the interval $[0, 2\pi]$.
Reveal solution
  1. Find $f'(x) = e^x \cos(x) - e^x \sin(x) = e^x(\cos(x) - \sin(x))$.
  2. Set $f'(x) = 0 \implies \tan(x) = 1$, so $x = \pi/4$ and $x = 5\pi/4$.
  3. Find $f''(x) = e^x(\cos(x) - \sin(x)) + e^x(-\sin(x) - \cos(x)) = -2e^x \sin(x)$.
  4. Set $f''(x) = 0 \implies \sin(x) = 0$, so $x = 0, \pi, 2\pi$.
  5. Determine concavity: concave down on $(0, \pi)$ and concave up on $(\pi, 2\pi)$.

Answer: Local max at $x=\pi/4$, local min at $x=5\pi/4$, inflection point at $x=\pi$.

Problem 3. Analyze and sketch $f(x) = x \cos(x)$ on the interval $[0, \pi]$.
Reveal solution
  1. Find $f'(x) = \cos(x) - x \sin(x)$.
  2. Set $f'(x) = 0 \implies \cot(x) = x$. This occurs at $x \approx 0.86$ radians.
  3. Find $f''(x) = -\sin(x) - (\sin(x) + x \cos(x)) = -2\sin(x) - x \cos(x)$.
  4. Set $f''(x) = 0 \implies \tan(x) = -x/2$. This occurs at $x \approx 2.28$ radians.
  5. Check endpoints: $f(0)=0$ and $f(\pi)=-\pi$.

Answer: Local max at $x \approx 0.86$, inflection point at $x \approx 2.28$, decreasing from the max to the endpoint.

L55. Optimisation I — method and classics

Module 2  2026-09-20  ·  11:58  ·  practice PDF

Practice problems (3)
Problem 1. A rectangle has a perimeter of $120$ cm. Find the dimensions that maximize its area.
Reveal solution
  1. Let $L$ be length and $W$ be width. The constraint is $2L + 2W = 120$, so $L = 60 - W$.
  2. The objective function is $A = L \times W = (60 - W)W = 60W - W^2$.
  3. Find the derivative: $A'(W) = 60 - 2W$.
  4. Set to zero: $60 - 2W = 0 \implies W = 30$.
  5. Then $L = 60 - 30 = 30$.
  6. Check endpoints $W=0$ and $W=60$, both give $A=0$. The maximum is at $W=30, L=30$.

Answer: L = 30 \text{ cm}, W = 30 \text{ cm}

Problem 2. A rectangular garden is to be fenced on three sides, with the fourth side being a wall. If there are $100$ meters of fencing, what is the maximum area?
Reveal solution
  1. Let $W$ be the two sides perpendicular to the wall and $L$ be the side parallel to the wall.
  2. Constraint: $L + 2W = 100 \implies L = 100 - 2W$.
  3. Objective: $A = W(100 - 2W) = 100W - 2W^2$.
  4. Derivative: $A'(W) = 100 - 4W$.
  5. Critical point: $100 - 4W = 0 \implies W = 25$.
  6. Calculate area: $A(25) = 25(100 - 50) = 25 \times 50 = 1250$.

Answer: 1250 \text{ square meters}

Problem 3. An open-top box is made from a $10 \times 10$ inch square of cardboard by cutting squares of side $x$ from the corners. Find $x$ to maximize volume.
Reveal solution
  1. The height is $x$ and the base sides are $10 - 2x$.
  2. Objective: $V(x) = x(10 - 2x)^2 = x(100 - 40x + 4x^2) = 4x^3 - 40x^2 + 100x$.
  3. Derivative: $V'(x) = 12x^2 - 80x + 100$.
  4. Set to zero and divide by $4$: $3x^2 - 20x + 25 = 0$.
  5. Factor: $(3x - 5)(x - 5) = 0$.
  6. Critical points are $x = 5/3$ and $x = 5$.
  7. Domain is $0 < x < 5$. At $x=5$, $V=0$. At $x=5/3$, $V = (5/3)(10 - 10/3)^2 = (5/3)(20/3)^2 = 2000/27$.

Answer: x = 5/3 \text{ inches}

L56. Optimisation II — harder problems + module review

Module 2  2026-09-21  ·  10:08  ·  practice PDF

Practice problems (3)
Problem 1. A rectangular box with a square base and an open top must have a volume of $10$ cubic meters. Find the dimensions that minimize the surface area.
Reveal solution
  1. Let $x$ be the side of the square base and $h$ be the height. The volume is $V = x^2 h = 10$, so $h = 10/x^2$.
  2. The surface area is $A = x^2 + 4xh$. Substituting $h$, we get $A(x) = x^2 + 4x(10/x^2) = x^2 + 40/x$.
  3. Find the derivative: $A'(x) = 2x - 40/x^2$.
  4. Set $A'(x) = 0$: $2x = 40/x^2$, which means $x^3 = 20$, so $x = \sqrt[3]{20}$.
  5. Find $h$: $h = 10/(\sqrt[3]{20})^2 = 10/20^{2/3} = (10 \cdot 20^{1/3})/20 = 20^{1/3}/2 = \sqrt[3]{20}/2$.

Answer: x = \sqrt[3]{20}, h = \frac{\sqrt[3]{20}}{2}

Problem 2. A piece of wire of length $20$ cm is cut into two pieces. One piece is bent into a square and the other into a circle. Where should the cut be made to minimize the total area enclosed?
Reveal solution
  1. Let $x$ be the length of the wire used for the square. The remaining $20-x$ is used for the circle.
  2. Side of square is $x/4$, area is $(x/4)^2 = x^2/16$.
  3. Circumference of circle is $2\pi r = 20-x$, so $r = (20-x)/(2\pi)$, area is $\pi((20-x)/(2\pi))^2 = (20-x)^2/(4\pi)$.
  4. Total area $A(x) = x^2/16 + (20-x)^2/(4\pi)$.
  5. Derivative $A'(x) = x/8 - (20-x)/(2\pi)$.
  6. Set $A'(x) = 0$: $x/8 = (20-x)/(2\pi) \implies \pi x = 4(20-x) \implies x(\pi+4) = 80 \implies x = 80/(\pi+4)$.

Answer: x = \frac{80}{\pi+4}

Problem 3. A light ray travels from a medium where its speed is $v_1$ to a medium where its speed is $v_2$. If the angle of incidence is $30^\circ$ and the angle of refraction is $45^\circ$, find the ratio $v_1/v_2$.
Reveal solution
  1. Use Snell's law: $\sin(\theta_1)/v_1 = \sin(\theta_2)/v_2$.
  2. Rearrange for the ratio: $v_1/v_2 = \sin(\theta_1)/\sin(\theta_2)$.
  3. Substitute the angles: $v_1/v_2 = \sin(30^\circ)/\sin(45^\circ)$.
  4. Calculate values: $\sin(30^\circ) = 1/2$ and $\sin(45^\circ) = \sqrt{2}/2$.
  5. Ratio: $(1/2) / (\sqrt{2}/2) = 1/\sqrt{2} = \sqrt{2}/2$.

Answer: \frac{\sqrt{2}}{2}

Module 3 — Integral Calculus (28/28)

L57. The area problem; Riemann sums

Module 3  2026-09-22  ·  9:53  ·  practice PDF

Practice problems (3)
Problem 1. Find the right Riemann sum $R_4$ for $f(x) = 3x + 2$ on the interval $[0, 2]$ with $n=4$ sub intervals.
Reveal solution
  1. Calculate width: $\Delta x = \frac{2-0}{4} = 0.5$.
  2. Identify right endpoints: $x_1=0.5, x_2=1.0, x_3=1.5, x_4=2.0$.
  3. Evaluate function: $f(0.5)=3.5, f(1.0)=5, f(1.5)=6.5, f(2.0)=8$.
  4. Sum and multiply: $R_4 = 0.5 \times (3.5 + 5 + 6.5 + 8) = 0.5 \times 23 = 11.5$.

Answer: 11.5

Problem 2. Approximate the area under $f(x) = \frac{1}{x}$ on the interval $[1, 3]$ using a left Riemann sum $L_2$ with $n=2$ sub intervals.
Reveal solution
  1. Calculate width: $\Delta x = \frac{3-1}{2} = 1$.
  2. Identify left endpoints: $x_0=1, x_1=2$.
  3. Evaluate function: $f(1)=1, f(2)=0.5$.
  4. Sum and multiply: $L_2 = 1 \times (1 + 0.5) = 1.5$.

Answer: 1.5

Problem 3. For $f(x) = x^3$ on $[0, 1]$ with $n=2$, calculate both $L_2$ and $R_2$. Which one is the overestimate and why?
Reveal solution
  1. Calculate width: $\Delta x = \frac{1-0}{2} = 0.5$.
  2. Left endpoints: $0, 0.5$. $L_2 = 0.5 \times (0^3 + 0.5^3) = 0.5 \times 0.125 = 0.0625$.
  3. Right endpoints: $0.5, 1$. $R_2 = 0.5 \times (0.5^3 + 1^3) = 0.5 \times 1.125 = 0.5625$.
  4. Since $f(x)=x^3$ is increasing on $[0, 1]$, the right sum $R_2$ is the overestimate.

Answer: L_2 = 0.0625, R_2 = 0.5625; R_2 is the overestimate because the function is increasing.

L58. The definite integral as a limit of sums

Module 3  2026-09-23  ·  10:54  ·  practice PDF

Practice problems (3)
Problem 1. Express the area under the curve $f(x) = 2x - 4$ from $x = 1$ to $x = 5$ as a definite integral. Based on the graph of the function, will the result be positive, negative, or zero?
Reveal solution
  1. The function is $f(x) = 2x - 4$.
  2. The interval is from $a = 1$ to $b = 5$.
  3. The definite integral is $\int_{1}^{5} (2x - 4) \, dx$.
  4. The function crosses the x axis at $2x - 4 = 0$, which is $x = 2$.
  5. From $x = 1$ to $x = 2$, the function is negative. From $x = 2$ to $x = 5$, the function is positive.
  6. The positive region (from 2 to 5) is larger than the negative region (from 1 to 2), so the result will be positive.

Answer: \int_{1}^{5} (2x - 4) \, dx; \text{Positive}

Problem 2. A function $f(x)$ is odd, meaning $f(-x) = -f(x)$. What is the value of the definite integral $\int_{-a}^{a} f(x) \, dx$ for any $a > 0$? Explain using the concept of signed area.
Reveal solution
  1. An odd function has rotational symmetry about the origin.
  2. This means for every point $(x, y)$ on the graph, there is a corresponding point $(-x, -y)$.
  3. The area between the curve and the x axis from $0$ to $a$ is exactly the same size as the area from $-a$ to $0$.
  4. However, because the function is odd, one of these areas is above the x axis and the other is below.
  5. The signed area from $-a$ to $0$ is the negative of the signed area from $0$ to $a$.
  6. Adding them together results in zero.

Answer: 0

Problem 3. Consider the function $f(x) = |x - 3|$ on the interval $[0, 6]$. Write the definite integral for the signed area and explain why the signed area is equal to the total geometric area in this specific case.
Reveal solution
  1. The definite integral is $\int_{0}^{6} |x - 3| \, dx$.
  2. The absolute value function $|x - 3|$ is always greater than or equal to zero for all $x$.
  3. Since $f(x) \ge 0$ for the entire interval $[0, 6]$, there is no area below the x axis.
  4. The signed area formula is $\text{Area}_{\text{above}} - \text{Area}_{\text{below}}$.
  5. Since $\text{Area}_{\text{below}} = 0$, the signed area is simply the area above the x axis.
  6. Therefore, the signed area equals the total geometric area.

Answer: \int_{0}^{6} |x - 3| \, dx; \text{The function is never negative.}

L59. Properties of definite integrals

Module 3  2026-09-24  ·  11:14  ·  practice PDF

Practice problems (3)
Problem 1. Use the properties of linearity to rewrite the integral $\int_{1}^{3} (5x^3 - 2x + 7) \, dx$ as a sum of three simpler integrals.
Reveal solution
  1. Apply the sum rule to split the integral into three parts: $\int_{1}^{3} 5x^3 \, dx - \int_{1}^{3} 2x \, dx + \int_{1}^{3} 7 \, dx$.
  2. Apply the constant multiple rule to move the coefficients outside: $5 \int_{1}^{3} x^3 \, dx - 2 \int_{1}^{3} x \, dx + 7 \int_{1}^{3} 1 \, dx$.

Answer: 5 \int_{1}^{3} x^3 \, dx - 2 \int_{1}^{3} x \, dx + 7 \int_{1}^{3} 1 \, dx

Problem 2. Given that $\int_{0}^{4} f(x) \, dx = 12$ and $\int_{0}^{2} f(x) \, dx = 5$, find the value of $\int_{2}^{4} f(x) \, dx$.
Reveal solution
  1. Use the additivity property: $\int_{0}^{4} f(x) \, dx = \int_{0}^{2} f(x) \, dx + \int_{2}^{4} f(x) \, dx$.
  2. Substitute the known values: $12 = 5 + \int_{2}^{4} f(x) \, dx$.
  3. Subtract 5 from both sides to isolate the unknown integral: $\int_{2}^{4} f(x) \, dx = 12 - 5 = 7$.

Answer: 7

Problem 3. If $f(x) \ge 3$ for all $x$ in the interval $[0, 5]$, what is the minimum possible value for $\int_{0}^{5} f(x) \, dx$?
Reveal solution
  1. Use the comparison property: since $f(x) \ge 3$, then $\int_{0}^{5} f(x) \, dx \ge \int_{0}^{5} 3 \, dx$.
  2. Calculate the integral of the constant function 3 over the interval $[0, 5]$.
  3. The area of a rectangle with height 3 and width $5 - 0 = 5$ is $3 \times 5 = 15$.

Answer: 15

L60. Antiderivatives and the indefinite integral

Module 3  2026-09-25  ·  10:55  ·  practice PDF

Practice problems (3)
Problem 1. Find the indefinite integral: $\int (5x^4 - 6x + 2) \, dx$.
Reveal solution
  1. Apply linearity to split the integral into three parts: $5 \int x^4 \, dx - 6 \int x \, dx + 2 \int 1 \, dx$.
  2. Use the power rule on $x^4$ to get $x^5/5$, so $5(x^5/5) = x^5$.
  3. Use the power rule on $x$ to get $x^2/2$, so $-6(x^2/2) = -3x^2$.
  4. The integral of the constant $2$ is $2x$.
  5. Combine the terms and add the constant of integration $C$.

Answer: x^5 - 3x^2 + 2x + C

Problem 2. Find the indefinite integral: $\int (\frac{3}{x^2} + e^x) \, dx$.
Reveal solution
  1. Rewrite the first term using a negative exponent: $\int (3x^{-2} + e^x) \, dx$.
  2. Apply the power rule to $3x^{-2}$: the new exponent is $-1$, and we divide by $-1$, giving $3(x^{-1}/-1) = -3x^{-1}$.
  3. The antiderivative of $e^x$ is $e^x$.
  4. Combine the results and add $C$.

Answer: -3x^{-1} + e^x + C

Problem 3. Find the indefinite integral: $\int (x+1)^2 \, dx$.
Reveal solution
  1. Expand the squared binomial first: $(x+1)^2 = x^2 + 2x + 1$.
  2. Set up the integral of the expanded form: $\int (x^2 + 2x + 1) \, dx$.
  3. Integrate $x^2$ to get $x^3/3$.
  4. Integrate $2x$ to get $x^2$.
  5. Integrate $1$ to get $x$.
  6. Combine all terms and add $C$.

Answer: \frac{1}{3}x^3 + x^2 + x + C

L61. The Fundamental Theorem of Calculus, part 1 — the bridge

Module 3  2026-09-26  ·  11:06  ·  practice PDF

Practice problems (3)
Problem 1. Find the derivative of the function $A(x) = \int_{2}^{x} (t^3 + 5t) \, dt$.
Reveal solution
  1. Identify the integrand as $f(t) = t^3 + 5t$.
  2. The upper limit of integration is simply $x$.
  3. Apply the Fundamental Theorem of Calculus part 1: $\frac{d}{dx} \int_{a}^{x} f(t) \, dt = f(x)$.
  4. Substitute $x$ for $t$ in the integrand.

Answer: x^3 + 5x

Problem 2. Compute the derivative of $G(x) = \int_{0}^{x^4} \sin(t) \, dt$.
Reveal solution
  1. Identify the integrand as $f(t) = \sin(t)$ and the upper limit as $g(x) = x^4$.
  2. Apply the chain rule version of the theorem: $f(g(x)) \cdot g'(x)$.
  3. The value of the integrand at the upper limit is $\sin(x^4)$.
  4. The derivative of the upper limit $x^4$ is $4x^3$.
  5. Multiply the two results together.

Answer: 4x^3 \sin(x^4)

Problem 3. Find the derivative of $H(x) = \int_{x}^{10} e^{t^2} \, dt$.
Reveal solution
  1. Notice that the variable $x$ is in the lower limit of integration.
  2. Use the property $\int_{x}^{a} f(t) \, dt = -\int_{a}^{x} f(t) \, dt$ to move $x$ to the upper limit.
  3. Rewrite the function as $H(x) = -\int_{10}^{x} e^{t^2} \, dt$.
  4. Apply the Fundamental Theorem of Calculus part 1 to the integral.
  5. The derivative is the negative of the integrand evaluated at $x$.

Answer: -e^{x^2}

L62. The Fundamental Theorem of Calculus, part 2 — proof and meaning

Module 3  2026-09-27  ·  10:52  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the definite integral $\int_{0}^{2} (x^3 + 2x) \, dx$.
Reveal solution
  1. Find the antiderivative of $x^3 + 2x$ using the reverse power rule: $F(x) = \frac{1}{4}x^4 + x^2$.
  2. Evaluate $F$ at the upper limit $x=2$: $F(2) = \frac{1}{4}(2)^4 + (2)^2 = \frac{16}{4} + 4 = 4 + 4 = 8$.
  3. Evaluate $F$ at the lower limit $x=0$: $F(0) = \frac{1}{4}(0)^4 + (0)^2 = 0$.
  4. Subtract the lower limit from the upper limit: $8 - 0 = 8$.

Answer: 8

Problem 2. Evaluate the definite integral $\int_{1}^{2} \frac{1}{x^2} \, dx$.
Reveal solution
  1. Rewrite the function as $x^{-2}$ to use the power rule.
  2. Find the antiderivative: $F(x) = \frac{x^{-1}}{-1} = -\frac{1}{x}$.
  3. Evaluate at the upper limit $x=2$: $F(2) = -\frac{1}{2}$.
  4. Evaluate at the lower limit $x=1$: $F(1) = -\frac{1}{1} = -1$.
  5. Subtract: $-\frac{1}{2} - (-1) = -\frac{1}{2} + 1 = \frac{1}{2}$.

Answer: 1/2

Problem 3. Evaluate the definite integral $\int_{0}^{\pi} \sin(x) \, dx$.
Reveal solution
  1. Find the antiderivative of $\sin(x)$. Since the derivative of $\cos(x)$ is $-\sin(x)$, the antiderivative is $F(x) = -\cos(x)$.
  2. Evaluate at the upper limit $x=\pi$: $F(\pi) = -\cos(\pi) = -(-1) = 1$.
  3. Evaluate at the lower limit $x=0$: $F(0) = -\cos(0) = -(1) = -1$.
  4. Subtract: $1 - (-1) = 2$.

Answer: 2

L63. Basic antiderivative catalogue; first practice clinic

Module 3  2026-09-28  ·  12:14  ·  practice PDF

Practice problems (3)
Problem 1. Find the indefinite integral: $\int (6x^5 - \frac{3}{x} + 2e^x) \, dx$
Reveal solution
  1. Apply linearity to split the integral into three parts: $6\int x^5 \, dx - 3\int \frac{1}{x} \, dx + 2\int e^x \, dx$.
  2. Use the power rule for the first term: $6 \frac{x^6}{6} = x^6$.
  3. Use the logarithm rule for the second term: $-3 \ln|x|$.
  4. Use the exponential rule for the third term: $2e^x$.
  5. Combine the results and add the constant of integration $C$.

Answer: x^6 - 3\ln|x| + 2e^x + C

Problem 2. Find the indefinite integral: $\int (\frac{1}{x^3} + \cos x + 5^x) \, dx$
Reveal solution
  1. Rewrite the first term as a power: $\int (x^{-3} + \cos x + 5^x) \, dx$.
  2. Apply the power rule to $x^{-3}$: $\frac{x^{-2}}{-2} = -\frac{1}{2x^2}$.
  3. Apply the trig rule to $\cos x$: $\sin x$.
  4. Apply the general exponential rule to $5^x$: $\frac{5^x}{\ln 5}$.
  5. Combine and add $C$.

Answer: -\frac{1}{2x^2} + \sin x + \frac{5^x}{\ln 5} + C

Problem 3. Find the indefinite integral: $\int (\frac{2}{\sqrt{x}} - \sin x + \sec^2 x) \, dx$
Reveal solution
  1. Rewrite the first term as $2x^{-1/2}$.
  2. Integrate $2x^{-1/2}$ using the power rule: $2 \frac{x^{1/2}}{1/2} = 4\sqrt{x}$.
  3. Integrate $-\sin x$: the antiderivative of $\sin x$ is $-\cos x$, so $-\sin x$ becomes $-(-\cos x) = \cos x$.
  4. Integrate $\sec^2 x$: the antiderivative is $\tan x$.
  5. Combine and add $C$.

Answer: 4\sqrt{x} + \cos x + \tan x + C

L64. u-substitution I — intuition and mechanics

Module 3  2026-09-29  ·  11:08  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the indefinite integral $\int x^2 e^{x^3} \, dx$.
Reveal solution
  1. Let $u = x^3$. Then $\frac{du}{dx} = 3x^2$, which means $dx = \frac{du}{3x^2}$.
  2. Substitute into the integral: $\int x^2 e^u \frac{du}{3x^2} = \frac{1}{3} \int e^u \, du$.
  3. Using the known integral of $e^u$, we get $\frac{1}{3} e^u + C$.
  4. Back-substitute $u = x^3$ to get $\frac{1}{3} e^{x^3} + C$.

Answer: \frac{1}{3} e^{x^3} + C

Problem 2. Evaluate the indefinite integral $\int \sin(4x - 2) \, dx$.
Reveal solution
  1. Let $u = 4x - 2$. Then $du = 4 \, dx$, so $dx = \frac{1}{4} du$.
  2. Substitute into the integral: $\int \sin(u) \frac{1}{4} du = \frac{1}{4} \int \sin(u) \, du$.
  3. The integral of $\sin(u)$ is $-\cos(u)$, so we have $-\frac{1}{4} \cos(u) + C$.
  4. Back-substitute $u = 4x - 2$ to get $-\frac{1}{4} \cos(4x - 2) + C$.

Answer: -\frac{1}{4} \cos(4x - 2) + C

Problem 3. Evaluate the indefinite integral $\int \frac{e^x}{1 + e^x} \, dx$.
Reveal solution
  1. Let $u = 1 + e^x$. Then $du = e^x \, dx$.
  2. The numerator $e^x \, dx$ is exactly $du$.
  3. The integral becomes $\int \frac{1}{u} \, du = \ln|u| + C$.
  4. Back-substitute $u = 1 + e^x$ to get $\ln(1 + e^x) + C$ (absolute value is not needed as $1+e^x$ is always positive).

Answer: \ln(1 + e^x) + C

L65. u-substitution II — definite integrals and trickier cases

Module 3  2026-09-30  ·  11:27  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the definite integral $\int_{0}^{\pi/2} \sin x \cos^2 x \, dx$.
Reveal solution
  1. Let $u = \cos x$. Then $du = -\sin x \, dx$, which means $\sin x \, dx = -du$.
  2. Change the limits: when $x = 0$, $u = \cos(0) = 1$. When $x = \pi/2$, $u = \cos(\pi/2) = 0$.
  3. The integral becomes $\int_{1}^{0} u^2 (-du) = \int_{0}^{1} u^2 \, du$.
  4. Integrate $u^2$ to get $\frac{1}{3}u^3$ and evaluate from $0$ to $1$.
  5. $\frac{1}{3}(1)^3 - \frac{1}{3}(0)^3 = \frac{1}{3}$.

Answer: 1/3

Problem 2. Find the indefinite integral $\int e^{5x-2} \, dx$.
Reveal solution
  1. Let $u = 5x - 2$. Then $du = 5 \, dx$, which means $dx = \frac{1}{5} du$.
  2. Substitute into the integral: $\int e^u \cdot \frac{1}{5} du$.
  3. Pull out the constant: $\frac{1}{5} \int e^u \, du$.
  4. Integrate $e^u$ to get $\frac{1}{5} e^u + C$.
  5. Back-substitute $u = 5x - 2$ to get $\frac{1}{5} e^{5x-2} + C$.

Answer: \frac{1}{5} e^{5x-2} + C

Problem 3. Evaluate $\int_{0}^{1} x \sqrt{x+1} \, dx$ using the substitution $u = \sqrt{x+1}$.
Reveal solution
  1. Let $u = \sqrt{x+1}$. Then $u^2 = x + 1$, so $x = u^2 - 1$.
  2. Differentiate $u^2 = x + 1$ to get $2u \, du = dx$.
  3. Change limits: when $x = 0$, $u = \sqrt{1} = 1$. When $x = 1$, $u = \sqrt{2}$.
  4. Substitute everything: $\int_{1}^{\sqrt{2}} (u^2 - 1) u (2u \, du) = \int_{1}^{\sqrt{2}} (2u^4 - 2u^2) \, du$.
  5. Integrate: $[\frac{2}{5}u^5 - \frac{2}{3}u^3]_{1}^{\sqrt{2}}$.
  6. Evaluate: $(\frac{2}{5}(\sqrt{2})^5 - \frac{2}{3}(\sqrt{2})^3) - (\frac{2}{5} - \frac{2}{3}) = (\frac{8\sqrt{2}}{5} - \frac{4\sqrt{2}}{3}) - (\frac{6-10}{15}) = \frac{24\sqrt{2}-20\sqrt{2}}{15} + \frac{4}{15} = \frac{4\sqrt{2}+4}{15}$.

Answer: \frac{4\sqrt{2}+4}{15}

L66. Integrals of trig functions

Module 3  2026-10-01  ·  11:09  ·  practice PDF

Practice problems (3)
Problem 1. Find the indefinite integral $\int \sec^3 x \tan x \, dx$.
Reveal solution
  1. Rewrite the integrand as $\sec^2 x (\sec x \tan x)$.
  2. Let $u = \sec x$, then $du = \sec x \tan x \, dx$.
  3. The integral becomes $\int u^2 \, du$.
  4. Integrate to get $\frac{1}{3}u^3 + C$.
  5. Substitute back $u = \sec x$.

Answer: \frac{1}{3}\sec^3 x + C

Problem 2. Evaluate $\int \sin^5 x \, dx$.
Reveal solution
  1. Peel off one sine: $\int (\sin^2 x)^2 \sin x \, dx$.
  2. Use $\sin^2 x = 1 - \cos^2 x$ to get $\int (1 - \cos^2 x)^2 \sin x \, dx$.
  3. Let $u = \cos x$, then $du = -\sin x \, dx$.
  4. The integral becomes $-\int (1 - u^2)^2 \, du = -\int (1 - 2u^2 + u^4) \, du$.
  5. Integrate: $-(u - \frac{2}{3}u^3 + \frac{1}{5}u^5) + C$.
  6. Substitute back $u = \cos x$.

Answer: -\cos x + \frac{2}{3}\cos^3 x - \frac{1}{5}\cos^5 x + C

Problem 3. Find the indefinite integral $\int \cos^3 x \sin^3 x \, dx$.
Reveal solution
  1. Both powers are odd, so we can choose either. Let's peel one cosine: $\int \cos^2 x \sin^3 x \cos x \, dx$.
  2. Replace $\cos^2 x$ with $1 - \sin^2 x$: $\int (1 - \sin^2 x) \sin^3 x \cos x \, dx$.
  3. Let $u = \sin x$, then $du = \cos x \, dx$.
  4. The integral becomes $\int (1 - u^2) u^3 \, du = \int (u^3 - u^5) \, du$.
  5. Integrate: $\frac{1}{4}u^4 - \frac{1}{6}u^6 + C$.
  6. Substitute back $u = \sin x$.

Answer: \frac{1}{4}\sin^4 x - \frac{1}{6}\sin^6 x + C

L67. Integration by parts I — the formula and LIATE

Module 3  2026-10-02  ·  10:10  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int x \ln x \, dx$.
Reveal solution
  1. Using LIATE, Logarithmic comes before Algebraic, so let $u = \ln x$ and $dv = x \, dx$.
  2. Compute $du = \frac{1}{x} \, dx$ and $v = \frac{1}{2}x^2$.
  3. Apply the formula: $\int u \, dv = \frac{1}{2}x^2 \ln x - \int \frac{1}{2}x^2 \cdot \frac{1}{x} \, dx$.
  4. Simplify the integral: $\int \frac{1}{2}x \, dx = \frac{1}{4}x^2$.
  5. Combine results: $\frac{1}{2}x^2 \ln x - \frac{1}{4}x^2 + C$.

Answer: \frac{1}{2}x^2 \ln x - \frac{1}{4}x^2 + C

Problem 2. Evaluate the integral $\int x e^{2x} \, dx$.
Reveal solution
  1. Let $u = x$ and $dv = e^{2x} \, dx$.
  2. Compute $du = dx$ and $v = \frac{1}{2}e^{2x}$.
  3. Apply the formula: $\frac{1}{2}x e^{2x} - \int \frac{1}{2}e^{2x} \, dx$.
  4. Integrate the remaining part: $\int \frac{1}{2}e^{2x} \, dx = \frac{1}{4}e^{2x}$.
  5. Combine results: $\frac{1}{2}x e^{2x} - \frac{1}{4}e^{2x} + C$.

Answer: \frac{1}{2}x e^{2x} - \frac{1}{4}e^{2x} + C

Problem 3. Evaluate the integral $\int x^2 \cos x \, dx$.
Reveal solution
  1. Let $u = x^2$ and $dv = \cos x \, dx$, so $du = 2x \, dx$ and $v = \sin x$.
  2. First pass: $x^2 \sin x - \int 2x \sin x \, dx$.
  3. For the second integral, let $u = 2x$ and $dv = \sin x \, dx$, so $du = 2 \, dx$ and $v = -\cos x$.
  4. Second pass: $\int 2x \sin x \, dx = -2x \cos x - \int -2 \cos x \, dx = -2x \cos x + 2 \sin x$.
  5. Combine everything: $x^2 \sin x - (-2x \cos x + 2 \sin x) + C = x^2 \sin x + 2x \cos x - 2 \sin x + C$.

Answer: x^2 \sin x + 2x \cos x - 2 \sin x + C

L68. Integration by parts II — repeated and cyclic cases

Module 3  2026-10-03  ·  10:47  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the indefinite integral $\int x^3 e^x \, dx$.
Reveal solution
  1. Use integration by parts with $u = x^3$ and $dv = e^x dx$. This gives $x^3 e^x - \int 3x^2 e^x dx$.
  2. Apply integration by parts again to $\int 3x^2 e^x dx$ with $u = 3x^2$ and $dv = e^x dx$. This gives $3x^2 e^x - \int 6x e^x dx$.
  3. Apply integration by parts a third time to $\int 6x e^x dx$ with $u = 6x$ and $dv = e^x dx$. This gives $6x e^x - \int 6 e^x dx$.
  4. The final integral is $6 e^x$. Combine all terms: $x^3 e^x - (3x^2 e^x - (6x e^x - 6 e^x))$.
  5. Simplify the signs: $x^3 e^x - 3x^2 e^x + 6x e^x - 6 e^x + C$.

Answer: (x^3 - 3x^2 + 6x - 6)e^x + C

Problem 2. Evaluate the indefinite integral $\int e^{2x} \cos(x) \, dx$.
Reveal solution
  1. Let $I = \int e^{2x} \cos x dx$. Set $u = \cos x$ and $dv = e^{2x} dx$. Then $du = -\sin x dx$ and $v = \frac{1}{2} e^{2x}$.
  2. First pass: $I = \frac{1}{2} e^{2x} \cos x + \int \frac{1}{2} e^{2x} \sin x dx$.
  3. Second pass on $\int \frac{1}{2} e^{2x} \sin x dx$: set $u = \sin x$ and $dv = \frac{1}{2} e^{2x} dx$. Then $du = \cos x dx$ and $v = \frac{1}{4} e^{2x}$.
  4. The second integral is $\frac{1}{4} e^{2x} \sin x - \int \frac{1}{4} e^{2x} \cos x dx$.
  5. Substitute back: $I = \frac{1}{2} e^{2x} \cos x + \frac{1}{4} e^{2x} \sin x - \frac{1}{4} I$.
  6. Add $\frac{1}{4} I$ to both sides: $\frac{5}{4} I = e^{2x} (\frac{1}{2} \cos x + \frac{1}{4} \sin x)$.
  7. Multiply by $\frac{4}{5}$: $I = \frac{4}{5} e^{2x} (\frac{1}{2} \cos x + \frac{1}{4} \sin x) = \frac{e^{2x}}{5} (2 \cos x + \sin x) + C$.

Answer: \frac{e^{2x}}{5} (2 \cos x + \sin x) + C

Problem 3. Evaluate the indefinite integral $\int x^2 \sin(x) \, dx$.
Reveal solution
  1. Set $u = x^2$ and $dv = \sin x dx$. Then $du = 2x dx$ and $v = -\cos x$.
  2. First pass: $-x^2 \cos x + \int 2x \cos x dx$.
  3. Second pass on $\int 2x \cos x dx$: set $u = 2x$ and $dv = \cos x dx$. Then $du = 2 dx$ and $v = \sin x$.
  4. The second integral is $2x \sin x - \int 2 \sin x dx$.
  5. The final integral is $-2(-\cos x) = 2 \cos x$.
  6. Combine: $-x^2 \cos x + 2x \sin x + 2 \cos x + C$.

Answer: (2 - x^2)\cos x + 2x \sin x + C

L69. Trigonometric integrals (powers of sin and cos)

Module 3  2026-10-04  ·  12:16  ·  practice PDF

Practice problems (3)
Problem 1. Find the indefinite integral $\int \sin^5 x \, dx$.
Reveal solution
  1. Since the power is odd, peel off one $\sin x$ factor: $\int (\sin^2 x)^2 \sin x \, dx$.
  2. Use the identity $\sin^2 x = 1 - \cos^2 x$ to get $\int (1 - \cos^2 x)^2 \sin x \, dx$.
  3. Let $u = \cos x$, then $du = -\sin x \, dx$. The integral becomes $-\int (1 - u^2)^2 \, du$.
  4. Expand the binomial: $-\int (1 - 2u^2 + u^4) \, du$.
  5. Integrate term by term: $-(u - \frac{2}{3}u^3 + \frac{1}{5}u^5) + C$.
  6. Substitute back $u = \cos x$: $-\cos x + \frac{2}{3}\cos^3 x - \frac{1}{5}\cos^5 x + C$.

Answer: -\cos x + \frac{2}{3}\cos^3 x - \frac{1}{5}\cos^5 x + C

Problem 2. Evaluate the indefinite integral $\int \sin^3 x \cos^3 x \, dx$.
Reveal solution
  1. Both powers are odd, so we can choose either. Let's peel one $\cos x$ and let $u = \sin x$.
  2. The integral becomes $\int \sin^3 x (\cos^2 x) \cos x \, dx = \int \sin^3 x (1 - \sin^2 x) \cos x \, dx$.
  3. Substitute $u = \sin x$ and $du = \cos x \, dx$: $\int u^3(1 - u^2) \, du$.
  4. Distribute: $\int (u^3 - u^5) \, du$.
  5. Integrate: $\frac{1}{4}u^4 - \frac{1}{6}u^6 + C$.
  6. Substitute back: $\frac{1}{4}\sin^4 x - \frac{1}{6}\sin^6 x + C$.

Answer: \frac{1}{4}\sin^4 x - \frac{1}{6}\sin^6 x + C

Problem 3. Find the indefinite integral $\int \cos^4 x \, dx$.
Reveal solution
  1. The power is even, so use the half-angle identity: $\int (\frac{1 + \cos(2x)}{2})^2 \, dx$.
  2. Expand the square: $\int \frac{1 + 2\cos(2x) + \cos^2(2x)}{4} \, dx$.
  3. Split the integral: $\frac{1}{4} \int 1 \, dx + \frac{1}{2} \int \cos(2x) \, dx + \frac{1}{4} \int \cos^2(2x) \, dx$.
  4. Apply the half-angle identity again to $\cos^2(2x)$: $\int \frac{1 + \cos(4x)}{2} \, dx$.
  5. Integrate all parts: $\frac{1}{4}x + \frac{1}{4}\sin(2x) + \frac{1}{4}(\frac{1}{2}x + \frac{1}{8}\sin(4x)) + C$.
  6. Simplify: $\frac{1}{4}x + \frac{1}{4}\sin(2x) + \frac{1}{8}x + \frac{1}{32}\sin(4x) + C = \frac{3}{8}x + \frac{1}{4}\sin(2x) + \frac{1}{32}\sin(4x) + C$.

Answer: \frac{3}{8}x + \frac{1}{4}\sin(2x) + \frac{1}{32}\sin(4x) + C

L70. Trigonometric substitution I

Module 3  2026-10-05  ·  12:56  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the indefinite integral $\int \frac{1}{\sqrt{25 - x^2}} \, dx$.
Reveal solution
  1. Identify $a = 5$ and use the substitution $x = 5 \sin \theta$.
  2. Compute the differential $dx = 5 \cos \theta \, d\theta$.
  3. Substitute into the integral: $\int \frac{5 \cos \theta}{\sqrt{25 - 25 \sin^2 \theta}} \, d\theta = \int \frac{5 \cos \theta}{5 \cos \theta} \, d\theta$.
  4. Integrate $1 \, d\theta$ to get $\theta + C$.
  5. Back-substitute using $\sin \theta = x/5$, so $\theta = \arcsin(x/5)$.

Answer: \arcsin\left(\frac{x}{5}\right) + C

Problem 2. Evaluate the indefinite integral $\int \frac{1}{(4 - x^2)^{3/2}} \, dx$.
Reveal solution
  1. Use $x = 2 \sin \theta$, so $dx = 2 \cos \theta \, d\theta$.
  2. The term $(4 - x^2)^{3/2}$ becomes $(4 \cos^2 \theta)^{3/2} = 8 \cos^3 \theta$.
  3. The integral becomes $\int \frac{2 \cos \theta}{8 \cos^3 \theta} \, d\theta = \frac{1}{4} \int \sec^2 \theta \, d\theta$.
  4. Integrate to get $\frac{1}{4} \tan \theta + C$.
  5. Using the triangle, $\tan \theta = \frac{x}{\sqrt{4 - x^2}}$.

Answer: \frac{x}{4\sqrt{4 - x^2}} + C

Problem 3. Evaluate the indefinite integral $\int \sqrt{9 - x^2} \, dx$.
Reveal solution
  1. Use $x = 3 \sin \theta$, so $dx = 3 \cos \theta \, d\theta$.
  2. The integral becomes $\int (3 \cos \theta)(3 \cos \theta) \, d\theta = 9 \int \cos^2 \theta \, d\theta$.
  3. Use the identity $\cos^2 \theta = \frac{1 + \cos(2\theta)}{2}$ to get $\frac{9}{2} \int (1 + \cos(2\theta)) \, d\theta$.
  4. Integrate to get $\frac{9}{2} \theta + \frac{9}{4} \sin(2\theta) + C$.
  5. Convert back: $\theta = \arcsin(x/3)$ and $\sin(2\theta) = 2(x/3)(\sqrt{9-x^2}/3) = \frac{2x\sqrt{9-x^2}}{9}$.

Answer: \frac{9}{2}\arcsin\left(\frac{x}{3}\right) + \frac{x\sqrt{9 - x^2}}{2} + C

L71. Trigonometric substitution II

Module 3  2026-10-06  ·  11:58  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int \frac{1}{x^2 + 25} \, dx$.
Reveal solution
  1. Identify the form $\sqrt{a^2 + x^2}$ with $a = 5$.
  2. Substitute $x = 5 \tan \theta$, so $dx = 5 \sec^2 \theta \, d\theta$.
  3. The denominator becomes $25 \tan^2 \theta + 25 = 25 \sec^2 \theta$.
  4. The integral becomes $\int \frac{5 \sec^2 \theta}{25 \sec^2 \theta} \, d\theta = \int \frac{1}{5} \, d\theta = \frac{1}{5} \theta + C$.
  5. Since $\tan \theta = \frac{x}{5}$, then $\theta = \arctan(\frac{x}{5})$.

Answer: \frac{1}{5} \arctan\left(\frac{x}{5}\right) + C

Problem 2. Evaluate the integral $\int \frac{1}{\sqrt{x^2 - 9}} \, dx$.
Reveal solution
  1. Identify the form $\sqrt{x^2 - a^2}$ with $a = 3$.
  2. Substitute $x = 3 \sec \theta$, so $dx = 3 \sec \theta \tan \theta \, d\theta$.
  3. The square root becomes $\sqrt{9 \sec^2 \theta - 9} = 3 \tan \theta$.
  4. The integral becomes $\int \frac{3 \sec \theta \tan \theta}{3 \tan \theta} \, d\theta = \int \sec \theta \, d\theta$.
  5. The antiderivative of $\sec \theta$ is $\ln|\sec \theta + \tan \theta| + C$.
  6. Substitute back $\sec \theta = \frac{x}{3}$ and $\tan \theta = \frac{\sqrt{x^2 - 9}}{3}$, giving $\ln|\frac{x + \sqrt{x^2 - 9}}{3}| + C$.

Answer: \ln\left|\frac{x + \sqrt{x^2 - 9}}{3}\right| + C

Problem 3. Evaluate the integral $\int \frac{x^3}{\sqrt{x^2 + 4}} \, dx$.
Reveal solution
  1. Substitute $x = 2 \tan \theta$, so $dx = 2 \sec^2 \theta \, d\theta$.
  2. The square root is $2 \sec \theta$ and $x^3 = 8 \tan^3 \theta$.
  3. The integral becomes $\int \frac{8 \tan^3 \theta}{2 \sec \theta} (2 \sec^2 \theta) \, d\theta = 8 \int \tan^3 \theta \sec \theta \, d\theta$.
  4. Rewrite as $8 \int \tan \theta (\tan^2 \theta \sec \theta) \, d\theta = 8 \int \tan \theta (\sec^2 \theta - 1) \sec \theta \, d\theta$.
  5. Alternatively, use $u = \sec \theta$, then $du = \sec \theta \tan \theta \, d\theta$.
  6. The integral is $8 \int (u^2 - 1) \, du = 8 (\frac{u^3}{3} - u) + C = 8 (\frac{\sec^3 \theta}{3} - \sec \theta) + C$.
  7. Substitute back $\sec \theta = \frac{\sqrt{x^2 + 4}}{2}$.

Answer: \frac{1}{3}(x^2+4)^{3/2} - 4\sqrt{x^2+4} + C

L72. Partial fractions I — distinct linear factors

Module 3  2026-10-07  ·  12:21  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int \frac{3x+1}{x^2-x-6} \, dx$.
Reveal solution
  1. Factor the denominator: $x^2-x-6 = (x-3)(x+2)$.
  2. Set up decomposition: $\frac{3x+1}{(x-3)(x+2)} = \frac{A}{x-3} + \frac{B}{x+2}$.
  3. Multiply by denominator: $3x+1 = A(x+2) + B(x-3)$.
  4. Find $A$ by letting $x=3$: $3(3)+1 = A(3+2) \implies 10 = 5A \implies A=2$.
  5. Find $B$ by letting $x=-2$: $3(-2)+1 = B(-2-3) \implies -5 = -5B \implies B=1$.
  6. Integrate: $\int (\frac{2}{x-3} + \frac{1}{x+2}) \, dx = 2\ln|x-3| + \ln|x+2| + C$.

Answer: 2\ln|x-3| + \ln|x+2| + C

Problem 2. Evaluate the integral $\int \frac{1}{x^2-4x} \, dx$.
Reveal solution
  1. Factor the denominator: $x^2-4x = x(x-4)$.
  2. Set up decomposition: $\frac{1}{x(x-4)} = \frac{A}{x} + \frac{B}{x-4}$.
  3. Multiply by denominator: $1 = A(x-4) + Bx$.
  4. Find $A$ by letting $x=0$: $1 = A(-4) \implies A = -1/4$.
  5. Find $B$ by letting $x=4$: $1 = B(4) \implies B = 1/4$.
  6. Integrate: $\int (\frac{-1/4}{x} + \frac{1/4}{x-4}) \, dx = \frac{1}{4}(\ln|x-4| - \ln|x|) + C$.

Answer: \frac{1}{4}\ln|\frac{x-4}{x}| + C

Problem 3. Evaluate the integral $\int \frac{x^2+1}{x^3-x} \, dx$.
Reveal solution
  1. Factor the denominator: $x^3-x = x(x-1)(x+1)$.
  2. Set up decomposition: $\frac{x^2+1}{x(x-1)(x+1)} = \frac{A}{x} + \frac{B}{x-1} + \frac{C}{x+1}$.
  3. Multiply by denominator: $x^2+1 = A(x-1)(x+1) + Bx(x+1) + Cx(x-1)$.
  4. Find $A$ by letting $x=0$: $1 = A(-1)(1) \implies A = -1$.
  5. Find $B$ by letting $x=1$: $1^2+1 = B(1)(2) \implies 2 = 2B \implies B = 1$.
  6. Find $C$ by letting $x=-1$: $(-1)^2+1 = C(-1)(-2) \implies 2 = 2C \implies C = 1$.
  7. Integrate: $\int (\frac{-1}{x} + \frac{1}{x-1} + \frac{1}{x+1}) \, dx = -\ln|x| + \ln|x-1| + \ln|x+1| + C$.

Answer: \ln|\frac{x^2-1}{x}| + C

L73. Partial fractions II — repeated and quadratic factors

Module 3  2026-10-08  ·  10:45  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int \frac{1}{x^2-4x+4} \, dx$.
Reveal solution
  1. Factor the denominator: $x^2-4x+4 = (x-2)^2$.
  2. Set up partial fractions: $\frac{1}{(x-2)^2} = \frac{A}{x-2} + \frac{B}{(x-2)^2}$.
  3. Solve for constants: $1 = A(x-2) + B$. Setting $x=2$ gives $B=1$. Comparing coefficients of $x$ gives $A=0$.
  4. Integrate: $\int \frac{1}{(x-2)^2} \, dx = -(x-2)^{-1} + C$.

Answer: -\frac{1}{x-2} + C

Problem 2. Evaluate the integral $\int \frac{1}{x^3+x} \, dx$.
Reveal solution
  1. Factor the denominator: $x(x^2+1)$.
  2. Set up partial fractions: $\frac{1}{x(x^2+1)} = \frac{A}{x} + \frac{Bx+C}{x^2+1}$.
  3. Solve for constants: $1 = A(x^2+1) + (Bx+C)x$.
  4. Comparing coefficients: $x^2$ terms: $A+B=0$; $x$ terms: $C=0$; constants: $A=1$. Thus $B=-1$.
  5. Integrate: $\int (\frac{1}{x} - \frac{x}{x^2+1}) \, dx = \ln|x| - \frac{1}{2}\ln(x^2+1) + C$.

Answer: \ln|x| - \frac{1}{2}\ln(x^2+1) + C

Problem 3. Evaluate the integral $\int \frac{x^2}{x^2+1} \, dx$.
Reveal solution
  1. Check degree: Numerator degree is 2, denominator degree is 2. The fraction is improper.
  2. Perform long division: $\frac{x^2}{x^2+1} = 1 - \frac{1}{x^2+1}$.
  3. Integrate the resulting terms: $\int 1 \, dx - \int \frac{1}{x^2+1} \, dx$.
  4. Apply basic integration rules: $x - \arctan(x) + C$.

Answer: x - \arctan(x) + C

L74. Rationalising substitutions and Weierstrass t = tan(x/2)

Module 3  2026-10-09  ·  13:02  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int \frac{1}{1 - \cos x} \, dx$.
Reveal solution
  1. Substitute $\cos x = \frac{1-t^2}{1+t^2}$ and $dx = \frac{2}{1+t^2} dt$.
  2. The denominator becomes $1 - \frac{1-t^2}{1+t^2} = \frac{1+t^2 - (1-t^2)}{1+t^2} = \frac{2t^2}{1+t^2}$.
  3. The integral is $\int \frac{1}{2t^2 / (1+t^2)} \cdot \frac{2}{1+t^2} \, dt = \int \frac{1}{t^2} \, dt$.
  4. Integrating $t^{-2}$ gives $-t^{-1} + C = -\frac{1}{t} + C$.
  5. Substitute back $t = \tan(x/2)$ to get $-\cot(x/2) + C$.

Answer: -\cot\left(\frac{x}{2}\right) + C

Problem 2. Evaluate the integral $\int \frac{1}{2 + \cos x} \, dx$.
Reveal solution
  1. Substitute $\cos x = \frac{1-t^2}{1+t^2}$ and $dx = \frac{2}{1+t^2} dt$.
  2. The denominator is $2 + \frac{1-t^2}{1+t^2} = \frac{2(1+t^2) + 1-t^2}{1+t^2} = \frac{3+t^2}{1+t^2}$.
  3. The integral becomes $\int \frac{1+t^2}{3+t^2} \cdot \frac{2}{1+t^2} \, dt = \int \frac{2}{3+t^2} \, dt$.
  4. This is an arctangent form: $2 \cdot \frac{1}{\sqrt{3}} \arctan\left(\frac{t}{\sqrt{3}}\right) + C$.
  5. Substitute back $t = \tan(x/2)$.

Answer: \frac{2}{\sqrt{3}} \arctan\left(\frac{\tan(x/2)}{\sqrt{3}}\right) + C

Problem 3. Evaluate the integral $\int \frac{\sin x}{1 + \sin x + \cos x} \, dx$.
Reveal solution
  1. Substitute $\sin x = \frac{2t}{1+t^2}$, $\cos x = \frac{1-t^2}{1+t^2}$, and $dx = \frac{2}{1+t^2} dt$.
  2. The integral becomes $\int \frac{2t/(1+t^2)}{1 + (2t + 1 - t^2)/(1+t^2)} \cdot \frac{2}{1+t^2} \, dt$.
  3. Simplify the denominator: $1 + \frac{2t+1-t^2}{1+t^2} = \frac{1+t^2+2t+1-t^2}{1+t^2} = \frac{2+2t}{1+t^2}$.
  4. The integral is $\int \frac{2t}{1+t^2} \cdot \frac{1+t^2}{2+2t} \cdot \frac{2}{1+t^2} \, dt = \int \frac{2t}{(1+t)(1+t^2)} \, dt$.
  5. Use partial fractions: $\frac{2t}{(1+t)(1+t^2)} = \frac{A}{1+t} + \frac{Bt+C}{1+t^2}$. Solving gives $A=-1, B=1, C=1$.
  6. Integrate: $\int (\frac{-1}{1+t} + \frac{t}{1+t^2} + \frac{1}{1+t^2}) \, dt = -\ln|1+t| + \frac{1}{2}\ln(1+t^2) + \arctan(t) + C$.

Answer: -\ln|1 + \tan(x/2)| + \frac{1}{2}\ln(1 + \tan^2(x/2)) + \frac{x}{2} + C

L75. Strategy: choosing the right technique

Module 3  2026-10-10  ·  12:52  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int x^2 \cos x \, dx$.
Reveal solution
  1. Identify the integrand as a product of an algebraic function $x^2$ and a trigonometric function $\cos x$.
  2. Since there is no clear u-substitution (the derivative of $x^2$ is $2x$, not $\cos x$), use integration by parts.
  3. Set $u = x^2$ and $dv = \cos x \, dx$, which gives $du = 2x \, dx$ and $v = \sin x$.
  4. Apply the formula: $\int x^2 \cos x \, dx = x^2 \sin x - \int 2x \sin x \, dx$.
  5. Apply integration by parts again to $\int 2x \sin x \, dx$ with $u = 2x$ and $dv = \sin x \, dx$.
  6. This gives $2x(-\cos x) - \int 2(-\cos x) \, dx = -2x \cos x + 2 \sin x$.

Answer: x^2 \sin x + 2x \cos x - 2 \sin x + C

Problem 2. Evaluate the integral $\int \frac{x+5}{x^2+2x-3} \, dx$.
Reveal solution
  1. Check for u-substitution: the derivative of $x^2+2x-3$ is $2x+2$, which is not a multiple of $x+5$.
  2. Recognize this as a rational function. Factor the denominator: $x^2+2x-3 = (x+3)(x-1)$.
  3. Use partial fraction decomposition: $\frac{x+5}{(x+3)(x-1)} = \frac{A}{x+3} + \frac{B}{x-1}$.
  4. Solve for A: $A = \frac{-3+5}{-3-1} = \frac{2}{-4} = -\frac{1}{2}$.
  5. Solve for B: $B = \frac{1+5}{1+3} = \frac{6}{4} = \frac{3}{2}$.
  6. Integrate: $\int (\frac{-1/2}{x+3} + \frac{3/2}{x-1}) \, dx = -\frac{1}{2}\ln|x+3| + \frac{3}{2}\ln|x-1| + C$.

Answer: \frac{3}{2}\ln|x-1| - \frac{1}{2}\ln|x+3| + C

Problem 3. Evaluate the integral $\int \frac{1}{\sqrt{9-x^2}} \, dx$.
Reveal solution
  1. Identify the form $\sqrt{a^2-x^2}$ in the denominator, where $a=3$.
  2. This is a cue for trigonometric substitution. Set $x = 3 \sin \theta$.
  3. Then $dx = 3 \cos \theta \, d\theta$ and $\sqrt{9-x^2} = \sqrt{9(1-\sin^2 \theta)} = 3 \cos \theta$.
  4. Substitute into the integral: $\int \frac{3 \cos \theta}{3 \cos \theta} \, d\theta = \int 1 \, d\theta = \theta + C$.
  5. Convert back to x: since $x/3 = \sin \theta$, then $\theta = \arcsin(x/3)$.

Answer: \arcsin\left(\frac{x}{3}\right) + C

L76. Mixed integration gauntlet I

Module 3  2026-10-11  ·  11:04  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the integral $\int x e^{x^2} \, dx$.
Reveal solution
  1. Identify that $x$ is a multiple of the derivative of the exponent $x^2$.
  2. Let $u = x^2$, then $du = 2x \, dx$, which means $x \, dx = \frac{1}{2} du$.
  3. Substitute into the integral: $\int \frac{1}{2} e^u \, du$.
  4. Integrate to get $\frac{1}{2} e^u + C$.
  5. Substitute back $u = x^2$.

Answer: \frac{1}{2} e^{x^2} + C

Problem 2. Evaluate the integral $\int \frac{1}{x^2-1} \, dx$.
Reveal solution
  1. Factor the denominator as $(x-1)(x+1)$.
  2. Use partial fraction decomposition: $\frac{1}{(x-1)(x+1)} = \frac{A}{x-1} + \frac{B}{x+1}$.
  3. Solve for constants: $1 = A(x+1) + B(x-1)$. Setting $x=1$ gives $A=1/2$; setting $x=-1$ gives $B=-1/2$.
  4. Integrate $\int (\frac{1/2}{x-1} - \frac{1/2}{x+1}) \, dx$.
  5. Result is $\frac{1}{2}\ln|x-1| - \frac{1}{2}\ln|x+1| + C$.

Answer: \frac{1}{2}\ln\left|\frac{x-1}{x+1}\right| + C

Problem 3. Evaluate the integral $\int x^3 \cos(x) \, dx$.
Reveal solution
  1. Use integration by parts with $u = x^3$ and $dv = \cos(x) \, dx$.
  2. First pass: $u=x^3, v=\sin(x)$. Integral becomes $x^3\sin(x) - \int 3x^2\sin(x) \, dx$.
  3. Second pass on $\int 3x^2\sin(x) \, dx$: $u=3x^2, v=-\cos(x)$. Integral becomes $-3x^2\cos(x) + \int 6x\cos(x) \, dx$.
  4. Third pass on $\int 6x\cos(x) \, dx$: $u=6x, v=\sin(x)$. Integral becomes $6x\sin(x) - \int 6\sin(x) \, dx$.
  5. Final integration: $- \int 6\sin(x) \, dx = 6\cos(x)$.
  6. Combine all terms: $x^3\sin(x) + 3x^2\cos(x) - 6x\sin(x) - 6\cos(x) + C$.

Answer: (x^3 - 6x)\sin(x) + (3x^2 - 6)\cos(x) + C

L77. Mixed integration gauntlet II

Module 3  2026-10-12  ·  12:14  ·  practice PDF

Practice problems (3)
Problem 1. Find the indefinite integral $\int x (\ln x)^2 \, dx$.
Reveal solution
  1. Use integration by parts with $u = (\ln x)^2$ and $dv = x \, dx$.
  2. Then $du = 2\ln x \cdot \frac{1}{x} \, dx$ and $v = \frac{1}{2}x^2$.
  3. The formula gives $\frac{1}{2}x^2(\ln x)^2 - \int \frac{1}{2}x^2 \cdot \frac{2\ln x}{x} \, dx$, which simplifies to $\frac{1}{2}x^2(\ln x)^2 - \int x \ln x \, dx$.
  4. Apply integration by parts again to $\int x \ln x \, dx$ with $u = \ln x$ and $dv = x \, dx$.
  5. This gives $\frac{1}{2}x^2 \ln x - \int \frac{1}{2}x \, dx = \frac{1}{2}x^2 \ln x - \frac{1}{4}x^2$.
  6. Combine all terms: $\frac{1}{2}x^2(\ln x)^2 - \frac{1}{2}x^2 \ln x + \frac{1}{4}x^2 + C$.

Answer: \frac{1}{2}x^2(\ln x)^2 - \frac{1}{2}x^2 \ln x + \frac{1}{4}x^2 + C

Problem 2. Evaluate the integral $\int \frac{1}{1 + \cos x} \, dx$.
Reveal solution
  1. Use the Weierstrass substitution $t = \tan(x/2)$.
  2. Substitute $\cos x = \frac{1-t^2}{1+t^2}$ and $dx = \frac{2}{1+t^2} \, dt$.
  3. The integral becomes $\int \frac{1}{1 + \frac{1-t^2}{1+t^2}} \cdot \frac{2}{1+t^2} \, dt$.
  4. Simplify the denominator: $1 + \frac{1-t^2}{1+t^2} = \frac{1+t^2+1-t^2}{1+t^2} = \frac{2}{1+t^2}$.
  5. The integral simplifies to $\int \frac{1+t^2}{2} \cdot \frac{2}{1+t^2} \, dt = \int 1 \, dt = t + C$.
  6. Substitute back $t = \tan(x/2)$.

Answer: \tan(x/2) + C

Problem 3. Find the antiderivative of $\int e^x \sin(2x) \, dx$.
Reveal solution
  1. Use integration by parts with $u = \sin(2x)$ and $dv = e^x \, dx$.
  2. Then $du = 2\cos(2x) \, dx$ and $v = e^x$.
  3. The first pass gives $e^x \sin(2x) - \int 2e^x \cos(2x) \, dx$.
  4. Apply integration by parts again to $\int 2e^x \cos(2x) \, dx$ with $u = 2\cos(2x)$ and $dv = e^x \, dx$.
  5. Then $du = -4\sin(2x) \, dx$ and $v = e^x$.
  6. This gives $2e^x \cos(2x) - \int -4e^x \sin(2x) \, dx = 2e^x \cos(2x) + 4\int e^x \sin(2x) \, dx$.
  7. Let $I = \int e^x \sin(2x) \, dx$. We have $I = e^x \sin(2x) - (2e^x \cos(2x) + 4I)$.
  8. Solve for $I$: $5I = e^x \sin(2x) - 2e^x \cos(2x) \implies I = \frac{e^x}{5}(\sin(2x) - 2\cos(2x)) + C$.

Answer: \frac{e^x}{5}(\sin(2x) - 2\cos(2x)) + C

L78. Improper integrals I — infinite limits

Module 3  2026-10-13  ·  10:44  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the improper integral $\int_{1}^{\infty} \frac{1}{x^3} \, dx$. Does it converge or diverge?
Reveal solution
  1. Rewrite as a limit: $\lim_{R \to \infty} \int_{1}^{R} x^{-3} \, dx$.
  2. Find the antiderivative: $\int x^{-3} \, dx = \frac{x^{-2}}{-2} = -\frac{1}{2x^2}$.
  3. Evaluate from $1$ to $R$: $\lim_{R \to \infty} [-\frac{1}{2R^2} - (-\frac{1}{2(1)^2})]$.
  4. Take the limit: $0 + \frac{1}{2} = \frac{1}{2}$.

Answer: Converges to $\frac{1}{2}$

Problem 2. Determine if the integral $\int_{1}^{\infty} \frac{1}{x^2+1} \, dx$ converges or diverges. Find its value if it converges.
Reveal solution
  1. Rewrite as a limit: $\lim_{R \to \infty} \int_{1}^{R} \frac{1}{x^2+1} \, dx$.
  2. The antiderivative is $\arctan(x)$.
  3. Evaluate from $1$ to $R$: $\lim_{R \to \infty} [\arctan(R) - \arctan(1)]$.
  4. The limit of $\arctan(R)$ as $R \to \infty$ is $\pi/2$.
  5. The value of $\arctan(1)$ is $\pi/4$.
  6. Result: $\pi/2 - \pi/4 = \pi/4$.

Answer: Converges to $\frac{\pi}{4}$

Problem 3. Evaluate $\int_{0}^{\infty} x e^{-x} \, dx$.
Reveal solution
  1. Rewrite as $\lim_{R \to \infty} \int_{0}^{R} x e^{-x} \, dx$.
  2. Use integration by parts: $u=x, dv=e^{-x}dx \implies du=dx, v=-e^{-x}$.
  3. Integral is $[-xe^{-x}]_0^R - \int_0^R -e^{-x} dx = [-xe^{-x} - e^{-x}]_0^R$.
  4. Evaluate: $\lim_{R \to \infty} (-Re^{-R} - e^{-R}) - (0 - e^0)$.
  5. Since $Re^{-R} \to 0$ and $e^{-R} \to 0$ as $R \to \infty$, we are left with $0 - (-1) = 1$.

Answer: Converges to $1$

L79. Improper integrals II — discontinuous integrands

Module 3  2026-10-14  ·  11:14  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the improper integral $\int_{0}^{1} \frac{1}{x^{3/2}} \, dx$. Does it converge or diverge?
Reveal solution
  1. Identify the discontinuity at $x=0$.
  2. Rewrite as a limit: $\lim_{t \to 0^{+}} \int_{t}^{1} x^{-3/2} \, dx$.
  3. Find the antiderivative: $\int x^{-3/2} \, dx = -2x^{-1/2} = -\frac{2}{\sqrt{x}}$.
  4. Evaluate the limit: $\lim_{t \to 0^{+}} [-\frac{2}{\sqrt{1}} - (-\frac{2}{\sqrt{t}})] = \lim_{t \to 0^{+}} (-2 + \frac{2}{\sqrt{t}})$.
  5. As $t \to 0^{+}$, the term $\frac{2}{\sqrt{t}}$ approaches infinity.

Answer: Diverges

Problem 2. Determine if the integral $\int_{0}^{1} \frac{1}{\sqrt{x} + x^2} \, dx$ converges or diverges using the comparison test.
Reveal solution
  1. Observe that for $x \in (0, 1]$, $\sqrt{x} + x^2 > \sqrt{x}$.
  2. Therefore, $\frac{1}{\sqrt{x} + x^2} < \frac{1}{\sqrt{x}}$.
  3. The integral $\int_{0}^{1} \frac{1}{\sqrt{x}} \, dx$ is a p-integral with $p = 1/2$.
  4. Since $p < 1$, the integral $\int_{0}^{1} \frac{1}{\sqrt{x}} \, dx$ converges.
  5. By the comparison test, since the original function is smaller than a convergent function, it also converges.

Answer: Converges

Problem 3. Evaluate $\int_{-1}^{1} \frac{1}{\sqrt[3]{x}} \, dx$.
Reveal solution
  1. Identify the discontinuity at $x=0$. Split the integral: $\int_{-1}^{0} x^{-1/3} \, dx + \int_{0}^{1} x^{-1/3} \, dx$.
  2. Evaluate the right side: $\lim_{t \to 0^{+}} [\frac{3}{2}x^{2/3}]_{t}^{1} = \frac{3}{2}(1)^{2/3} - 0 = \frac{3}{2}$.
  3. Evaluate the left side: $\lim_{t \to 0^{-}} [\frac{3}{2}x^{2/3}]_{-1}^{t} = 0 - \frac{3}{2}(-1)^{2/3} = -\frac{3}{2}$.
  4. Sum the two parts: $\frac{3}{2} + (-\frac{3}{2}) = 0$.

Answer: 0

L80. Numerical integration: trapezoid and Simpson's rule

Module 3  2026-10-15  ·  12:18  ·  practice PDF

Practice problems (3)
Problem 1. Use the trapezoid rule with $n=4$ to approximate the integral $\int_{1}^{2} \ln(x) \, dx$.
Reveal solution
  1. Calculate $h = (2-1)/4 = 0.25$.
  2. Find x values: $1, 1.25, 1.5, 1.75, 2$.
  3. Evaluate $f(x) = \ln(x)$: $\ln(1)=0, \ln(1.25) \approx 0.223, \ln(1.5) \approx 0.405, \ln(1.75) \approx 0.560, \ln(2) \approx 0.693$.
  4. Apply formula: $\frac{0.25}{2} [0 + 2(0.223 + 0.405 + 0.560) + 0.693]$.
  5. Compute: $0.125 [0 + 2(1.188) + 0.693] = 0.125 [2.376 + 0.693] = 0.125 [3.069] \approx 0.384$.

Answer: 0.384

Problem 2. Use Simpson's rule with $n=4$ to approximate the integral $\int_{0}^{1} \frac{1}{1+x^2} \, dx$.
Reveal solution
  1. Calculate $h = (1-0)/4 = 0.25$.
  2. Find x values: $0, 0.25, 0.5, 0.75, 1$.
  3. Evaluate $f(x) = 1/(1+x^2)$: $f(0)=1, f(0.25)=0.941, f(0.5)=0.8, f(0.75)=0.64, f(1)=0.5$.
  4. Apply formula: $\frac{0.25}{3} [1 + 4(0.941) + 2(0.8) + 4(0.64) + 0.5]$.
  5. Compute: $\frac{0.25}{3} [1 + 3.764 + 1.6 + 2.56 + 0.5] = \frac{0.25}{3} [9.424] \approx 0.785$.

Answer: 0.785

Problem 3. Explain why Simpson's rule is exact for the integral $\int_{0}^{2} (x^3 + 2x) \, dx$ regardless of the even number $n$ chosen, and calculate the exact value using the Fundamental Theorem of Calculus to verify.
Reveal solution
  1. Simpson's rule is based on quadratic interpolation. The error term for Simpson's rule involves the fourth derivative of the function.
  2. For $f(x) = x^3 + 2x$, the first derivative is $3x^2 + 2$, the second is $6x$, the third is $6$, and the fourth is $0$.
  3. Since the fourth derivative is zero, the error is zero, making the approximation exact.
  4. Using FTC: $\int_{0}^{2} (x^3 + 2x) \, dx = [\frac{1}{4}x^4 + x^2]_{0}^{2} = (\frac{16}{4} + 4) - (0) = 4 + 4 = 8$.

Answer: 8

L81. The integral of sec x — the historical puzzle

Module 3  2026-10-16  ·  11:42  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the indefinite integral $\int 5 \sec x \, dx$.
Reveal solution
  1. Use the linearity of the integral to pull the constant 5 outside: $5 \int \sec x \, dx$.
  2. Apply the standard formula $\int \sec x \, dx = \ln |\sec x + \tan x|$.
  3. Combine the results and add the constant of integration.

Answer: $5 \ln |\sec x + \tan x| + C$

Problem 2. Evaluate the definite integral $\int_{0}^{\pi/6} \sec x \, dx$.
Reveal solution
  1. Find the antiderivative: $\ln |\sec x + \tan x|$.
  2. Evaluate at the upper limit $\pi/6$: $\sec(\pi/6) = 2/\sqrt{3}$ and $\tan(\pi/6) = 1/\sqrt{3}$.
  3. The sum is $3/\sqrt{3} = \sqrt{3}$, so the value is $\ln(\sqrt{3})$.
  4. Evaluate at the lower limit $0$: $\sec(0) = 1$ and $\tan(0) = 0$, giving $\ln(1) = 0$.
  5. Subtract the lower limit from the upper limit.

Answer: $\ln(\sqrt{3})$

Problem 3. Evaluate the indefinite integral $\int \sec(3x) \, dx$.
Reveal solution
  1. Use u-substitution: let $u = 3x$, then $du = 3 \, dx$, which means $dx = \frac{1}{3} du$.
  2. Rewrite the integral as $\frac{1}{3} \int \sec u \, du$.
  3. Apply the formula: $\frac{1}{3} \ln |\sec u + \tan u| + C$.
  4. Substitute $3x$ back in for $u$.

Answer: $\frac{1}{3} \ln |\sec(3x) + \tan(3x)| + C$

L82. Average value of a function; MVT for integrals

Module 3  2026-10-17  ·  11:48  ·  practice PDF

Practice problems (3)
Problem 1. Find the average value of $f(x) = 4x^3 - 2x$ on the interval $[0, 2]$.
Reveal solution
  1. Identify the interval width: $b - a = 2 - 0 = 2$.
  2. Set up the integral: $\int_{0}^{2} (4x^3 - 2x) \, dx$.
  3. Find the antiderivative: $x^4 - x^2$.
  4. Evaluate the definite integral: $(2^4 - 2^2) - (0^4 - 0^2) = 16 - 4 = 12$.
  5. Divide by the width: $f_{\text{avg}} = \frac{12}{2} = 6$.

Answer: 6

Problem 2. Find the average value of $f(x) = \frac{1}{x}$ on the interval $[1, e]$.
Reveal solution
  1. Identify the interval width: $e - 1$.
  2. Set up the integral: $\int_{1}^{e} \frac{1}{x} \, dx$.
  3. Find the antiderivative: $\ln |x|$.
  4. Evaluate the definite integral: $\ln(e) - \ln(1) = 1 - 0 = 1$.
  5. Divide by the width: $f_{\text{avg}} = \frac{1}{e-1}$.

Answer: \frac{1}{e-1}

Problem 3. For $f(x) = 3x^2$ on the interval $[0, 2]$, find the average value and then find the value $c$ guaranteed by the Mean Value Theorem for Integrals.
Reveal solution
  1. Interval width is $2 - 0 = 2$.
  2. Integral is $\int_{0}^{2} 3x^2 \, dx = [x^3]_{0}^{2} = 8 - 0 = 8$.
  3. Average value is $f_{\text{avg}} = \frac{8}{2} = 4$.
  4. Set $f(c) = 4$, so $3c^2 = 4$.
  5. Solve for $c$: $c^2 = \frac{4}{3}$, so $c = \sqrt{\frac{4}{3}} = \frac{2}{\sqrt{3}}$ (since $c$ must be in $[0, 2]$).

Answer: f_{\text{avg}} = 4, c = \frac{2}{\sqrt{3}}

L83. Integrals with symmetry — even/odd shortcuts and the king property

Module 3  2026-10-18  ·  10:42  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the definite integral $\int_{-\pi}^{\pi} x \sin(x) \, dx$ using symmetry.
Reveal solution
  1. Check the symmetry of $f(x) = x \sin(x)$.
  2. Substitute $-x$: $f(-x) = (-x) \sin(-x)$.
  3. Since $\sin(-x) = -\sin(x)$, we have $f(-x) = (-x)(-\sin(x)) = x \sin(x)$.
  4. The function is even, so the integral is $2 \int_{0}^{\pi} x \sin(x) \, dx$.
  5. Use integration by parts: $\int x \sin(x) \, dx = -x \cos(x) + \sin(x)$.
  6. Evaluate from $0$ to $\pi$: $[-\pi \cos(\pi) + \sin(\pi)] - [0 + 0] = [-\pi(-1) + 0] = \pi$.
  7. Multiply by $2$ to get $2\pi$.

Answer: 2\pi

Problem 2. Evaluate $\int_{-1}^{1} (x^5 + x^3 + x) \cos(x) \, dx$.
Reveal solution
  1. Let $f(x) = (x^5 + x^3 + x) \cos(x)$.
  2. Check symmetry: $f(-x) = ((-x)^5 + (-x)^3 + (-x)) \cos(-x)$.
  3. This simplifies to $(-x^5 - x^3 - x) \cos(x) = -(x^5 + x^3 + x) \cos(x)$.
  4. Since $f(-x) = -f(x)$, the function is odd.
  5. The integral of an odd function over a symmetric interval $[-1, 1]$ is $0$.

Answer: 0

Problem 3. Use the King Property to evaluate $\int_{0}^{\pi/4} \frac{1}{1 + \tan(x)} \, dx$.
Reveal solution
  1. Let $I = \int_{0}^{\pi/4} \frac{1}{1 + \tan(x)} \, dx$.
  2. Apply the King Property: replace $x$ with $\pi/4 - x$.
  3. Use the identity $\tan(\pi/4 - x) = \frac{1 - \tan(x)}{1 + \tan(x)}$.
  4. The integrand becomes $\frac{1}{1 + \frac{1 - \tan(x)}{1 + \tan(x)}} = \frac{1 + \tan(x)}{(1 + \tan(x)) + (1 - \tan(x))} = \frac{1 + \tan(x)}{2}$.
  5. Add the two versions of $I$: $2I = \int_{0}^{\pi/4} (\frac{1}{1 + \tan(x)} + \frac{1 + \tan(x)}{2}) \, dx$.
  6. Alternatively, rewrite the integrand as $\frac{\cos(x)}{\cos(x) + \sin(x)}$.
  7. By King Property, $I = \int_{0}^{\pi/4} \frac{\sin(x)}{\sin(x) + \cos(x)} \, dx$.
  8. Then $2I = \int_{0}^{\pi/4} 1 \, dx = \pi/4$.
  9. Thus $I = \pi/8$.

Answer: \pi/8

L84. Module 3 review + self-test

Module 3  2026-10-19  ·  11:36  ·  practice PDF

Practice problems (3)
Problem 1. Evaluate the indefinite integral $\int x^2 \cos x \, dx$.
Reveal solution
  1. Identify this as a product of a polynomial and a trig function, suggesting integration by parts.
  2. Let $u = x^2$ and $dv = \cos x \, dx$. Then $du = 2x \, dx$ and $v = \sin x$.
  3. Apply the formula: $\int x^2 \cos x \, dx = x^2 \sin x - \int 2x \sin x \, dx$.
  4. Apply integration by parts again to $\int 2x \sin x \, dx$ with $u = 2x$ and $dv = \sin x \, dx$.
  5. Then $du = 2 \, dx$ and $v = -\cos x$.
  6. The second integral becomes $-2x \cos x - \int -2 \cos x \, dx = -2x \cos x + 2 \sin x$.
  7. Combine everything: $x^2 \sin x - (-2x \cos x + 2 \sin x) + C = x^2 \sin x + 2x \cos x - 2 \sin x + C$.

Answer: x^2 \sin x + 2x \cos x - 2 \sin x + C

Problem 2. Evaluate the definite integral $\int_{0}^{1} \frac{1}{1+x^2} \, dx$.
Reveal solution
  1. Recognize the integrand as the derivative of the arctangent function.
  2. The antiderivative of $\frac{1}{1+x^2}$ is $\arctan(x)$.
  3. Evaluate at the boundaries: $\arctan(1) - \arctan(0)$.
  4. Since $\tan(\pi/4) = 1$ and $\tan(0) = 0$, the values are $\pi/4$ and $0$.

Answer: \frac{\pi}{4}

Problem 3. Evaluate the indefinite integral $\int \frac{1}{x^2 - 5x + 6} \, dx$.
Reveal solution
  1. Factor the denominator: $x^2 - 5x + 6 = (x-2)(x-3)$.
  2. Use partial fraction decomposition: $\frac{1}{(x-2)(x-3)} = \frac{A}{x-2} + \frac{B}{x-3}$.
  3. Solve for constants: $1 = A(x-3) + B(x-2)$.
  4. Setting $x=3$ gives $B=1$. Setting $x=2$ gives $A=-1$.
  5. The integral becomes $\int \frac{-1}{x-2} \, dx + \int \frac{1}{x-3} \, dx$.
  6. Integrate to get $-\ln|x-2| + \ln|x-3| + C = \ln\left|\frac{x-3}{x-2}\right| + C$.

Answer: \ln\left|\frac{x-3}{x-2}\right| + C

Module 4 — Applications of Integration (12/16)

L85. Area between curves

Module 4  2026-10-20  ·  13:30  ·  practice PDF

Practice problems (3)
Problem 1. Find the area of the region enclosed by the curve $y = x^2 - 4$ and the x-axis ($y = 0$).
Reveal solution
  1. Find intersections: $x^2 - 4 = 0 \implies x = \pm 2$.
  2. Determine top/bottom: For $x=0$, $y = -4$, so the x-axis ($y=0$) is the top boundary.
  3. Set up integral: $\int_{-2}^{2} (0 - (x^2 - 4)) \, dx = \int_{-2}^{2} (4 - x^2) \, dx$.
  4. Integrate: $[4x - \frac{1}{3}x^3]_{-2}^{2}$.
  5. Evaluate: $(8 - \frac{8}{3}) - (-8 + \frac{8}{3}) = \frac{16}{3} - (-\frac{16}{3}) = \frac{32}{3}$.

Answer: $\frac{32}{3}$

Problem 2. Find the area of the region enclosed by $y = \sqrt{x}$ and $y = x^2$.
Reveal solution
  1. Find intersections: $\sqrt{x} = x^2 \implies x = x^4 \implies x(x^3 - 1) = 0 \implies x = 0, 1$.
  2. Determine top/bottom: For $x=0.25$, $\sqrt{0.25} = 0.5$ and $0.25^2 = 0.0625$. So $\sqrt{x}$ is on top.
  3. Set up integral: $\int_{0}^{1} (\sqrt{x} - x^2) \, dx$.
  4. Integrate: $[\frac{2}{3}x^{3/2} - \frac{1}{3}x^3]_{0}^{1}$.
  5. Evaluate: $(\frac{2}{3} - \frac{1}{3}) - 0 = \frac{1}{3}$.

Answer: $\frac{1}{3}$

Problem 3. Find the area of the region enclosed by $x = y^2 - 1$ and $x = 1 - y^2$.
Reveal solution
  1. Find intersections: $y^2 - 1 = 1 - y^2 \implies 2y^2 = 2 \implies y^2 = 1 \implies y = \pm 1$.
  2. Determine right/left: For $y=0$, $x = -1$ for the first curve and $x = 1$ for the second. So $1 - y^2$ is on the right.
  3. Set up integral: $\int_{-1}^{1} ((1 - y^2) - (y^2 - 1)) \, dy = \int_{-1}^{1} (2 - 2y^2) \, dy$.
  4. Integrate: $[2y - \frac{2}{3}y^3]_{-1}^{1}$.
  5. Evaluate: $(2 - \frac{2}{3}) - (-2 + \frac{2}{3}) = \frac{4}{3} - (-\frac{4}{3}) = \frac{8}{3}$.

Answer: $\frac{8}{3}$

L86. Volumes by slicing (discs and washers)

Module 4  2026-10-21  ·  10:49  ·  practice PDF

Practice problems (3)
Problem 1. Find the volume of the solid generated by rotating the region bounded by $y = x^3$, the $x$-axis, and $x = 2$ around the $x$-axis.
Reveal solution
  1. Identify the radius: $R(x) = x^3$.
  2. Set up the integral: $V = \int_{0}^{2} \pi (x^3)^2 \, dx = \pi \int_{0}^{2} x^6 \, dx$.
  3. Integrate: $\pi [\frac{1}{7}x^7]_{0}^{2}$.
  4. Evaluate: $\pi (\frac{128}{7} - 0) = \frac{128\pi}{7}$.

Answer: $\frac{128\pi}{7}$

Problem 2. Find the volume of the solid formed by rotating the region bounded by $y = x^2$ and $y = 4$ around the $y$-axis.
Reveal solution
  1. Solve for $x$ in terms of $y$: $x = \sqrt{y}$.
  2. Identify the radius: $R(y) = \sqrt{y}$.
  3. Set up the integral from $y=0$ to $y=4$: $V = \int_{0}^{4} \pi (\sqrt{y})^2 \, dy = \pi \int_{0}^{4} y \, dy$.
  4. Integrate and evaluate: $\pi [\frac{1}{2}y^2]_{0}^{4} = \pi (8 - 0) = 8\pi$.

Answer: $8\pi$

Problem 3. Find the volume of the solid formed by rotating the region bounded by $y = x$ and $y = \sqrt{x}$ around the $x$-axis.
Reveal solution
  1. Find intersection points: $x = \sqrt{x} \implies x^2 = x \implies x(x-1)=0$, so $x=0$ and $x=1$.
  2. Identify radii: Outer $R(x) = \sqrt{x}$, Inner $r(x) = x$.
  3. Set up the washer integral: $V = \pi \int_{0}^{1} ((\sqrt{x})^2 - x^2) \, dx = \pi \int_{0}^{1} (x - x^2) \, dx$.
  4. Integrate: $\pi [\frac{1}{2}x^2 - \frac{1}{3}x^3]_{0}^{1}$.
  5. Evaluate: $\pi (\frac{1}{2} - \frac{1}{3}) = \pi (\frac{3-2}{6}) = \frac{\pi}{6}$.

Answer: $\frac{\pi}{6}$

L87. Volumes by cylindrical shells

Module 4  2026-10-22  ·  10:28  ·  practice PDF

Practice problems (3)
Problem 1. Find the volume of the solid generated by rotating the region bounded by $y = x^3$, $y = 0$, and $x = 1$ around the $y$-axis using the shell method.
Reveal solution
  1. Identify the radius as $x$ and the height as $f(x) = x^3$.
  2. Set up the integral: $V = \int_{0}^{1} 2\pi x (x^3) \, dx$.
  3. Simplify the integrand: $V = 2\pi \int_{0}^{1} x^4 \, dx$.
  4. Integrate: $V = 2\pi [\frac{1}{5}x^5]_{0}^{1}$.
  5. Evaluate: $V = 2\pi (\frac{1}{5} - 0) = \frac{2\pi}{5}$.

Answer: $\frac{2\pi}{5}$

Problem 2. Find the volume of the solid generated by rotating the region bounded by $y = \sqrt{x}$, $y = 0$, and $x = 4$ around the $y$-axis using the shell method.
Reveal solution
  1. Identify the radius as $x$ and the height as $f(x) = \sqrt{x}$.
  2. Set up the integral: $V = \int_{0}^{4} 2\pi x (x^{1/2}) \, dx$.
  3. Simplify the integrand: $V = 2\pi \int_{0}^{4} x^{3/2} \, dx$.
  4. Integrate: $V = 2\pi [\frac{2}{5}x^{5/2}]_{0}^{4}$.
  5. Evaluate: $V = 2\pi (\frac{2}{5} \cdot 32) = 2\pi (\frac{64}{5}) = \frac{128\pi}{5}$.

Answer: $\frac{128\pi}{5}$

Problem 3. Find the volume of the solid generated by rotating the region bounded by $x = y^2$ and $x = 2y$ around the $y$-axis using the shell method.
Reveal solution
  1. Find intersection points: $y^2 = 2y \implies y(y-2) = 0$, so $y=0$ and $y=2$.
  2. The intersection points in x are $x=0$ and $x=4$.
  3. For rotation around the $y$-axis, shells use $dx$. The height is the difference between the upper curve $y = \sqrt{x}$ and the lower curve $y = x/2$.
  4. Set up the integral: $V = \int_{0}^{4} 2\pi x (\sqrt{x} - \frac{x}{2}) \, dx$.
  5. Simplify: $V = 2\pi \int_{0}^{4} (x^{3/2} - \frac{1}{2}x^2) \, dx$.
  6. Integrate: $V = 2\pi [\frac{2}{5}x^{5/2} - \frac{1}{6}x^3]_{0}^{4}$.
  7. Evaluate: $V = 2\pi (\frac{64}{5} - \frac{64}{6}) = 2\pi (\frac{384 - 320}{30}) = 2\pi (\frac{64}{30}) = \frac{64\pi}{15}$.

Answer: $\frac{64\pi}{15}$

L88. Which volume method? Technique clinic

Module 4  2026-10-23  ·  11:38  ·  practice PDF

Practice problems (3)
Problem 1. Find the volume of the solid formed by rotating the region bounded by $y = x^3$ and $y = x$ in the first quadrant around the x-axis.
Reveal solution
  1. Find intersection points: $x^3 = x \implies x(x^2-1) = 0$, so $x=0$ and $x=1$ in the first quadrant.
  2. Using the washer method (integrate $dx$): Outer radius $R(x) = x$, inner radius $r(x) = x^3$.
  3. Set up integral: $V = \int_{0}^{1} \pi (x^2 - (x^3)^2) \, dx = \pi \int_{0}^{1} (x^2 - x^6) \, dx$.
  4. Integrate: $\pi [\frac{1}{3}x^3 - \frac{1}{7}x^7]_0^1 = \pi (\frac{1}{3} - \frac{1}{7}) = \pi (\frac{7-3}{21}) = \frac{4\pi}{21}$.

Answer: $\frac{4\pi}{21}$

Problem 2. Find the volume of the solid formed by rotating the region bounded by $y = \sqrt{x}$, $y = 0$, and $x = 4$ around the y-axis.
Reveal solution
  1. Identify boundaries: $x$ goes from $0$ to $4$. $y$ goes from $0$ to $2$.
  2. Using shell method (integrate $dx$): Radius is $x$, height is $\sqrt{x}$.
  3. Set up integral: $V = \int_{0}^{4} 2\pi x \sqrt{x} \, dx = 2\pi \int_{0}^{4} x^{3/2} \, dx$.
  4. Integrate: $2\pi [\frac{2}{5}x^{5/2}]_0^4 = 2\pi (\frac{2}{5} \cdot 32) = \frac{128\pi}{5}$.

Answer: $\frac{128\pi}{5}$

Problem 3. A region is bounded by $y = x^2$ and $y = 2x$. Rotate this region around the line $x = -1$. Which method is more efficient, and what is the volume?
Reveal solution
  1. Intersection points: $x^2 = 2x \implies x=0, x=2$.
  2. Shell method (integrate $dx$): Radius is $x - (-1) = x+1$. Height is $2x - x^2$.
  3. Set up integral: $V = \int_{0}^{2} 2\pi (x+1)(2x - x^2) \, dx$.
  4. Expand: $(x+1)(2x - x^2) = 2x^2 - x^3 + 2x - x^2 = -x^3 + x^2 + 2x$.
  5. Integrate: $2\pi [-\frac{1}{4}x^4 + \frac{1}{3}x^3 + x^2]_0^2 = 2\pi (-4 + \frac{8}{3} + 4) = 2\pi (\frac{8}{3}) = \frac{16\pi}{3}$.

Answer: $\frac{16\pi}{3}$

L89. Arc length

Module 4  2026-10-24  ·  10:30  ·  practice PDF

Practice problems (3)
Problem 1. Find the arc length of the curve $y = \frac{x^3}{6} + \frac{1}{2x}$ on the interval $[1, 2]$.
Reveal solution
  1. Compute the derivative: $y' = \frac{3x^2}{6} - \frac{1}{2x^2} = \frac{x^2}{2} - \frac{1}{2x^2}$.
  2. Compute $1 + (y')^2$: $1 + (\frac{x^4}{4} - \frac{1}{2} + \frac{1}{4x^4}) = \frac{x^4}{4} + \frac{1}{2} + \frac{1}{4x^4}$.
  3. Recognize the perfect square: $1 + (y')^2 = (\frac{x^2}{2} + \frac{1}{2x^2})^2$.
  4. Integrate: $L = \int_{1}^{2} (\frac{x^2}{2} + \frac{1}{2x^2}) \, dx = [\frac{x^3}{6} - \frac{1}{2x}]_{1}^{2}$.
  5. Evaluate: $(\frac{8}{6} - \frac{1}{4}) - (\frac{1}{6} - \frac{1}{2}) = \frac{7}{6} + \frac{1}{4} = \frac{14+3}{12} = \frac{17}{12}$.

Answer: 17/12

Problem 2. Find the length of the curve $y = \frac{2}{3}x^{3/2}$ from $x = 0$ to $x = 3$.
Reveal solution
  1. Compute the derivative: $y' = \frac{2}{3} \cdot \frac{3}{2} x^{1/2} = x^{1/2}$.
  2. Set up the integral: $L = \int_{0}^{3} \sqrt{1 + (x^{1/2})^2} \, dx = \int_{0}^{3} \sqrt{1 + x} \, dx$.
  3. Use u-substitution where $u = 1+x$: $L = [\frac{2}{3}(1+x)^{3/2}]_{0}^{3}$.
  4. Evaluate: $\frac{2}{3}(4)^{3/2} - \frac{2}{3}(1)^{3/2} = \frac{2}{3}(8) - \frac{2}{3}(1) = \frac{14}{3}$.

Answer: 14/3

Problem 3. A curve is defined by $y = \ln(\cos x)$ on the interval $[0, \pi/4]$. Find its arc length.
Reveal solution
  1. Compute the derivative: $y' = \frac{1}{\cos x} \cdot (-\sin x) = -\tan x$.
  2. Set up the integral: $L = \int_{0}^{\pi/4} \sqrt{1 + (-\tan x)^2} \, dx$.
  3. Use the identity $1 + \tan^2 x = \sec^2 x$: $L = \int_{0}^{\pi/4} \sqrt{\sec^2 x} \, dx = \int_{0}^{\pi/4} \sec x \, dx$.
  4. Integrate: $L = [\ln|\sec x + \tan x|]_{0}^{\pi/4}$.
  5. Evaluate: $\ln|\sec(\pi/4) + \tan(\pi/4)| - \ln|\sec(0) + \tan(0)| = \ln|\sqrt{2} + 1| - \ln|1 + 0| = \ln(1 + \sqrt{2})$.

Answer: \ln(1 + \sqrt{2})

L90. Surface area of revolution

Module 4  2026-10-25  ·  14:04  ·  practice PDF

Practice problems (3)
Problem 1. Find the surface area of the surface generated by rotating the curve $y = x^2$ from $x = 0$ to $x = 1$ around the $y$-axis.
Reveal solution
  1. Identify the radius for rotation around the $y$-axis: $r = x$.
  2. Find the derivative: $y' = 2x$.
  3. Set up the arc length element: $ds = \sqrt{1 + (2x)^2} dx = \sqrt{1 + 4x^2} dx$.
  4. Set up the integral: $S = \int_{0}^{1} 2\pi x \sqrt{1 + 4x^2} \, dx$.
  5. Use u-substitution: $u = 1 + 4x^2$, $du = 8x dx$, so $x dx = \frac{1}{8} du$.
  6. Integrate: $S = \frac{2\pi}{8} \int_{1}^{5} u^{1/2} du = \frac{\pi}{4} [\frac{2}{3}u^{3/2}]_{1}^{5} = \frac{\pi}{6}(5\sqrt{5}-1)$.

Answer: \frac{\pi}{6}(5\sqrt{5}-1)

Problem 2. Find the surface area of the surface generated by rotating the line $y = 2x$ from $x = 0$ to $x = 3$ around the $x$-axis.
Reveal solution
  1. Find the derivative: $y' = 2$.
  2. Calculate the arc length element: $\sqrt{1 + 2^2} = \sqrt{5}$.
  3. Set up the integral: $S = \int_{0}^{3} 2\pi (2x) \sqrt{5} \, dx$.
  4. Integrate: $S = 4\pi\sqrt{5} \int_{0}^{3} x \, dx = 4\pi\sqrt{5} [\frac{1}{2}x^2]_{0}^{3}$.
  5. Evaluate: $4\pi\sqrt{5} \cdot \frac{9}{2} = 18\pi\sqrt{5}$.

Answer: 18\pi\sqrt{5}

Problem 3. A curve is defined by $x = \sqrt{y}$ from $y = 0$ to $y = 4$. Find the surface area when this curve is rotated around the $y$-axis.
Reveal solution
  1. The radius is $x = \sqrt{y}$.
  2. Find the derivative of $x$ with respect to $y$: $x' = \frac{1}{2\sqrt{y}}$.
  3. Set up the integral: $S = \int_{0}^{4} 2\pi \sqrt{y} \sqrt{1 + \frac{1}{4y}} \, dy$.
  4. Simplify the integrand: $2\pi \sqrt{y} \frac{\sqrt{4y+1}}{2\sqrt{y}} = \pi \sqrt{4y+1}$.
  5. Integrate: $S = \pi \int_{0}^{4} (4y+1)^{1/2} \, dy = \pi [ \frac{1}{4} \cdot \frac{2}{3} (4y+1)^{3/2} ]_{0}^{4}$.
  6. Evaluate: $\frac{\pi}{6} (17^{3/2} - 1^{3/2}) = \frac{\pi}{6}(17\sqrt{17}-1)$.

Answer: \frac{\pi}{6}(17\sqrt{17}-1)

L91. Work, force, and fluid pressure

Module 4  2026-10-26  ·  11:23  ·  practice PDF

Practice problems (3)
Problem 1. A spring has a spring constant of $k = 500$ Newtons per meter. Find the work done in stretching the spring from $x = 0.1$ meters to $x = 0.3$ meters.
Reveal solution
  1. The force function is $F(x) = 500x$.
  2. The work is the integral of $F(x)$ from $0.1$ to $0.3$: $W = \int_{0.1}^{0.3} 500x \, dx$.
  3. The antiderivative is $250x^2$.
  4. Evaluate at the boundaries: $250(0.3)^2 - 250(0.1)^2 = 250(0.09) - 250(0.01)$.
  5. Calculate the final value: $22.5 - 2.5 = 20$.

Answer: 20 \text{ Joules}

Problem 2. A cylindrical tank of radius $1$ meter and height $4$ meters is full of water (density $\rho = 1000$ kg/m$^3$, $g = 9.8$ m/s$^2$). Find the work required to pump all the water out of the top.
Reveal solution
  1. The volume of a slice at height $x$ is $V = \pi(1)^2 dx = \pi dx$.
  2. The weight of the slice is $F = 1000 \cdot 9.8 \cdot \pi dx = 9800\pi dx$.
  3. The distance the slice is lifted is $4 - x$.
  4. The work integral is $W = \int_{0}^{4} 9800\pi (4-x) \, dx$.
  5. Integrate: $9800\pi [4x - \frac{1}{2}x^2]_{0}^{4} = 9800\pi (16 - 8) = 9800\pi (8)$.
  6. Final result: $78400\pi$.

Answer: 78400\pi \text{ Joules}

Problem 3. A vertical rectangular plate $2$ meters wide and $3$ meters high is submerged in water. The top of the plate is $1$ meter below the surface. Find the total fluid force on the plate (use $\rho g = 9800$ N/m$^3$).
Reveal solution
  1. Let $h$ be the depth from the surface. The plate extends from $h = 1$ to $h = 4$.
  2. The pressure at depth $h$ is $P = 9800h$.
  3. The area of a thin strip is $w \cdot dh = 2 \, dh$.
  4. The force integral is $F = \int_{1}^{4} (9800h)(2) \, dh = 19600 \int_{1}^{4} h \, dh$.
  5. Integrate: $19600 [\frac{1}{2}h^2]_{1}^{4} = 9800(4^2 - 1^2)$.
  6. Calculate: $9800(16 - 1) = 9800(15) = 147000$.

Answer: 147000 \text{ Newtons}

L92. Centre of mass and centroids

Module 4  2026-10-27  ·  10:50  ·  practice PDF

Practice problems (3)
Problem 1. Find the centroid of the region bounded by $y = \sqrt{x}$, the $x$-axis, and the line $x = 4$.
Reveal solution
  1. Calculate area: $A = \int_{0}^{4} x^{1/2} \, dx = [\frac{2}{3}x^{3/2}]_{0}^{4} = \frac{2}{3}(8) = \frac{16}{3}$.
  2. Calculate $M_y$: $M_y = \int_{0}^{4} x(x^{1/2}) \, dx = \int_{0}^{4} x^{3/2} \, dx = [\frac{2}{5}x^{5/2}]_{0}^{4} = \frac{2}{5}(32) = \frac{64}{5}$.
  3. Calculate $M_x$: $M_x = \int_{0}^{4} \frac{1}{2}(\sqrt{x})^2 \, dx = \int_{0}^{4} \frac{1}{2}x \, dx = [\frac{1}{4}x^2]_{0}^{4} = 4$.
  4. Find $\bar{x} = \frac{64/5}{16/3} = \frac{64}{5} \cdot \frac{3}{16} = \frac{12}{5} = 2.4$.
  5. Find $\bar{y} = \frac{4}{16/3} = 4 \cdot \frac{3}{16} = \frac{3}{4} = 0.75$.

Answer: (\frac{12}{5}, \frac{3}{4})

Problem 2. Find the centroid of the region bounded by $y = x^2$ and $y = x$.
Reveal solution
  1. Intersection points are $x=0$ and $x=1$.
  2. Area $A = \int_{0}^{1} (x - x^2) \, dx = [\frac{1}{2}x^2 - \frac{1}{3}x^3]_{0}^{1} = \frac{1}{6}$.
  3. Moment $M_y = \int_{0}^{1} x(x - x^2) \, dx = [\frac{1}{3}x^3 - \frac{1}{4}x^4]_{0}^{1} = \frac{1}{12}$.
  4. Moment $M_x = \int_{0}^{1} \frac{1}{2}(x^2 - (x^2)^2) \, dx = \frac{1}{2}[\frac{1}{3}x^3 - \frac{1}{5}x^5]_{0}^{1} = \frac{1}{2}(\frac{2}{15}) = \frac{1}{15}$.
  5. $\bar{x} = \frac{1/12}{1/6} = \frac{1}{2}$, $\bar{y} = \frac{1/15}{1/6} = \frac{2}{5}$.

Answer: (\frac{1}{2}, \frac{2}{5})

Problem 3. Find the centroid of the region bounded by $y = e^x$, $y = 0$, $x = 0$, and $x = 1$.
Reveal solution
  1. Area $A = \int_{0}^{1} e^x \, dx = [e^x]_{0}^{1} = e - 1$.
  2. Moment $M_y = \int_{0}^{1} x e^x \, dx$. Using integration by parts: $[xe^x - e^x]_{0}^{1} = (e - e) - (0 - 1) = 1$.
  3. Moment $M_x = \int_{0}^{1} \frac{1}{2}(e^x)^2 \, dx = \int_{0}^{1} \frac{1}{2}e^{2x} \, dx = [\frac{1}{4}e^{2x}]_{0}^{1} = \frac{1}{4}(e^2 - 1)$.
  4. $\bar{x} = \frac{1}{e-1}$.
  5. $\bar{y} = \frac{(e^2-1)/4}{e-1} = \frac{(e-1)(e+1)}{4(e-1)} = \frac{e+1}{4}$.

Answer: (\frac{1}{e-1}, \frac{e+1}{4})

L93. Gabriel's horn: finite volume, infinite surface

Module 4  2026-10-28  ·  9:52  ·  practice PDF

Practice problems (3)
Problem 1. Consider the solid formed by rotating $f(x) = \frac{1}{x^2}$ for $x \ge 1$ around the $x$-axis. Does the volume $V$ converge? If so, find its value.
Reveal solution
  1. Set up the volume integral: $V = \int_{1}^{\infty} \pi (\frac{1}{x^2})^2 \, dx = \int_{1}^{\infty} \frac{\pi}{x^4} \, dx$.
  2. Evaluate as a limit: $\lim_{b \to \infty} \int_{1}^{b} \pi x^{-4} \, dx$.
  3. Find the antiderivative: $\pi [\frac{x^{-3}}{-3}]_{1}^{b} = \pi [-\frac{1}{3b} - (-\frac{1}{3})]$.
  4. Take the limit as $b \to \infty$: $\pi (0 + \frac{1}{3}) = \frac{\pi}{3}$.

Answer: V = \frac{\pi}{3}

Problem 2. For the solid in the previous problem ($f(x) = \frac{1}{x^2}$ for $x \ge 1$), use a comparison to determine if the surface area $S$ is finite or infinite.
Reveal solution
  1. The surface area formula is $S = \int_{1}^{\infty} 2\pi \frac{1}{x^2} \sqrt{1 + (-\frac{2}{x^3})^2} \, dx$.
  2. Note that $\sqrt{1 + \frac{4}{x^6}} > 1$ for all $x \ge 1$.
  3. Therefore, $S > \int_{1}^{\infty} \frac{2\pi}{x^2} \, dx$.
  4. The integral $\int_{1}^{\infty} \frac{1}{x^2} \, dx$ is a p-integral with $p=2$, which converges.
  5. Since the integrand $\frac{1}{x^2} \sqrt{1 + \frac{4}{x^6}}$ behaves like $\frac{1}{x^2}$ for large $x$, the surface area actually converges.

Answer: The surface area is finite.

Problem 3. Suppose we rotate $f(x) = \frac{1}{\sqrt{x}}$ for $x \ge 1$ around the $x$-axis. Determine if the volume $V$ is finite or infinite.
Reveal solution
  1. Set up the volume integral: $V = \int_{1}^{\infty} \pi (\frac{1}{\sqrt{x}})^2 \, dx$.
  2. Simplify the integrand: $V = \int_{1}^{\infty} \frac{\pi}{x} \, dx$.
  3. Evaluate the improper integral: $\lim_{b \to \infty} [\pi \ln x]_{1}^{b}$.
  4. The limit $\lim_{b \to \infty} \pi \ln b$ diverges to infinity.

Answer: V = \infty

L94. Introduction to differential equations; separable ODEs

Module 4  2026-10-29  ·  9:49  ·  practice PDF

Practice problems (3)
Problem 1. Find the general solution for the differential equation $\frac{dy}{dx} = xy$.
Reveal solution
  1. Separate the variables by dividing by $y$ and multiplying by $dx$: $\frac{1}{y} \, dy = x \, dx$.
  2. Integrate both sides: $\int \frac{1}{y} \, dy = \int x \, dx$.
  3. Evaluate the integrals: $\ln|y| = \frac{1}{2}x^2 + C$.
  4. Exponentiate both sides: $|y| = e^{\frac{1}{2}x^2 + C} = e^C e^{\frac{1}{2}x^2}$.
  5. Replace $e^C$ with a constant $A$: $y = Ae^{\frac{1}{2}x^2}$.

Answer: $y = Ae^{\frac{1}{2}x^2}$

Problem 2. Solve the initial value problem $\frac{dy}{dx} = 3y$ with the condition $y(0) = 5$.
Reveal solution
  1. Separate the variables: $\frac{1}{y} \, dy = 3 \, dx$.
  2. Integrate both sides: $\ln|y| = 3x + C$.
  3. Solve for $y$: $y = Ae^{3x}$.
  4. Apply the initial condition $y(0) = 5$: $5 = Ae^{3(0)} \implies 5 = A(1) \implies A = 5$.
  5. Substitute $A$ back into the general solution: $y = 5e^{3x}$.

Answer: $y = 5e^{3x}$

Problem 3. Find the general solution for $\frac{dy}{dx} = \frac{x^2}{y^2}$.
Reveal solution
  1. Separate the variables: $y^2 \, dy = x^2 \, dx$.
  2. Integrate both sides: $\int y^2 \, dy = \int x^2 \, dx$.
  3. Evaluate the integrals: $\frac{1}{3}y^3 = \frac{1}{3}x^3 + C$.
  4. Multiply by 3 to simplify: $y^3 = x^3 + 3C$.
  5. Let $K = 3C$ and solve for $y$: $y = \sqrt[3]{x^3 + K}$.

Answer: $y = \sqrt[3]{x^3 + K}$

L95. Exponential growth and decay models

Module 4  2026-10-30  ·  10:27  ·  practice PDF

Practice problems (3)
Problem 1. A radioactive isotope has a half-life of $1600$ years. If a sample initially contains $50$ grams, how much remains after $2000$ years?
Reveal solution
  1. Find the decay constant $k$ using $k = \frac{-\ln 2}{1600}$.
  2. Set up the model $y(t) = 50 e^{kt}$.
  3. Substitute $t = 2000$ into the equation: $y(2000) = 50 e^{\frac{-\ln 2}{1600} \cdot 2000}$.
  4. Simplify the exponent: $y(2000) = 50 e^{-1.25 \ln 2} = 50(2^{-1.25})$.
  5. Calculate the final value: $y(2000) \approx 21.02$ grams.

Answer: 21.02 \text{ g}

Problem 2. A population of fish in a pond grows exponentially. Initially, there are $200$ fish. After $2$ years, there are $350$ fish. Find the doubling time of the population.
Reveal solution
  1. Use the model $y(t) = 200 e^{kt}$ and the condition $y(2) = 350$.
  2. Solve for $k$: $350 = 200 e^{2k} \implies 1.75 = e^{2k} \implies k = \frac{\ln(1.75)}{2} \approx 0.2798$.
  3. Use the doubling time formula $t_{\text{double}} = \frac{\ln 2}{k}$.
  4. Substitute $k$: $t_{\text{double}} = \frac{\ln 2}{0.2798} \approx 2.48$ years.

Answer: 2.48 \text{ years}

Problem 3. A metal rod at $150 ^\circ\text{C}$ is placed in a room at $20 ^\circ\text{C}$. After $5$ minutes, the rod is $100 ^\circ\text{C}$. How long will it take for the rod to reach $40 ^\circ\text{C}$?
Reveal solution
  1. Use Newton's Law of Cooling: $T(t) = 20 + (150 - 20)e^{kt} = 20 + 130 e^{kt}$.
  2. Find $k$ using $T(5) = 100$: $100 = 20 + 130 e^{5k} \implies 80 = 130 e^{5k} \implies e^{5k} = \frac{8}{13}$.
  3. Solve for $k$: $k = \frac{\ln(8/13)}{5} \approx -0.0937$.
  4. Set $T(t) = 40$ and solve for $t$: $40 = 20 + 130 e^{-0.0937t} \implies 20 = 130 e^{-0.0937t}$.
  5. Divide by $130$: $\frac{2}{13} = e^{-0.0937t} \implies \ln(2/13) = -0.0937t$.
  6. Calculate $t$: $t = \frac{\ln(2/13)}{-0.0937} \approx 19.4$ minutes.

Answer: 19.4 \text{ minutes}

L96. Logistic growth

Module 4  2026-10-31  ·  11:13  ·  practice PDF

Practice problems (3)
Problem 1. A population of fish in a pond follows the logistic model with a carrying capacity of $L = 2000$ and a growth constant $k = 0.1$ per month. If the initial population is $y(0) = 500$, find the population after $t = 10$ months.
Reveal solution
  1. Use the formula $y(t) = \frac{L}{1 + Ae^{-kt}}$.
  2. Find $A$ using $y(0) = 500$: $500 = \frac{2000}{1 + A} \implies 1 + A = 4 \implies A = 3$.
  3. Substitute $t = 10$: $y(10) = \frac{2000}{1 + 3e^{-0.1(10)}} = \frac{2000}{1 + 3e^{-1}}$.
  4. Calculate $e^{-1} \approx 0.3679$, so $y(10) \approx \frac{2000}{1 + 1.1037} \approx \frac{2000}{2.1037} \approx 950.7$.

Answer: $y(10) \approx 951$

Problem 2. A city's population grows according to the logistic equation $\frac{dy}{dt} = 0.02y(1 - \frac{y}{100000})$. If the current population is $20,000$, what is the intrinsic growth rate $k$ and the carrying capacity $L$?
Reveal solution
  1. Compare the given equation $\frac{dy}{dt} = 0.02y(1 - \frac{y}{100000})$ to the standard form $\frac{dy}{dt} = ky(1 - \frac{y}{L})$.
  2. The coefficient $k$ is the value multiplying the $y$ term outside the parentheses, so $k = 0.02$.
  3. The value $L$ is the denominator in the fraction inside the parentheses, so $L = 100,000$.

Answer: $k = 0.02, L = 100,000$

Problem 3. Show that for the logistic equation $\frac{dy}{dt} = ky(1 - \frac{y}{L})$, the maximum growth rate occurs when the population is exactly half of the carrying capacity, $y = L/2$.
Reveal solution
  1. The growth rate is given by the function $f(y) = ky - \frac{k}{L}y^2$.
  2. To find the maximum of $f(y)$, take the derivative with respect to $y$ and set it to zero: $f'(y) = k - \frac{2k}{L}y$.
  3. Set $k - \frac{2k}{L}y = 0$.
  4. Divide by $k$ (assuming $k \neq 0$): $1 - \frac{2y}{L} = 0 \implies \frac{2y}{L} = 1 \implies y = L/2$.

Answer: $y = L/2$

L97. First-order linear ODEs and integrating factors tomorrow 07:15
L98. Slope fields and qualitative behaviour
L99. Parametric curves: calculus along a path
L100. Polar coordinates: area and arc length + module review
Module 5 — Sequences & Series (0/24)
L101. Sequences: convergence and limits
L102. Series and partial sums; the geometric series
L103. The harmonic series diverges — three proofs
L104. The divergence (nth term) test
L105. Integral test and p-series
L106. Comparison and limit comparison tests
L107. Alternating series and the estimation theorem
L108. Absolute vs conditional convergence; rearrangement shock
L109. Ratio test
L110. Root test; strategy for choosing tests
L111. Series gauntlet — mixed practice
L112. Power series and radius of convergence
L113. Representing functions as power series
L114. Differentiating and integrating power series
L115. Taylor and Maclaurin series — the big idea
L116. Building the classic series: e^x, sin x, cos x
L117. The binomial series
L118. Taylor's theorem and error bounds
L119. Using series to compute limits and integrals
L120. Euler's formula
L121. The Basel problem
L122. Fourier series — a glimpse
L123. Series gauntlet II
L124. Module 5 review + self-test
Module 6 — Advanced Topics (0/24)
L125. Multivariable functions and partial derivatives
L126. Higher partials and Clairaut's theorem
L127. Gradient, directional derivatives, level curves
L128. Multivariable chain rule
L129. Optimisation in two variables; second derivative test
L130. Lagrange multipliers
L131. Double integrals over rectangles
L132. Double integrals over general regions; Fubini
L133. Double integrals in polar coordinates
L134. The Gaussian integral via polar coordinates
L135. Triple integrals; cylindrical and spherical coordinates
L136. Change of variables and the Jacobian
L137. Differentiation under the integral sign I
L138. Feynman's technique II — famous applications
L139. The Gamma function: extending the factorial
L140. Beta function and its relation to Gamma
L141. The Dirichlet integral
L142. Line integrals — a first look
L143. Green's theorem — a first look
L144. Laplace transforms I — definition and basic transforms
L145. Laplace transforms II — solving ODEs
L146. Where to go next: analysis, PDEs, complex analysis
L147. Grand review I — differential calculus in 15 minutes
L148. Grand review II — integral calculus in 15 minutes