Answer: $40$ cm³/s

Write the volume formula: $V = s^2 \cdot 10 = 10s^2$ since the height is constant. Differentiate and substitute: $\frac{dV}{dt} = 20s\frac{ds}{dt} = 20(4)(0.5) = 40$ cm³/s. Why distractors fail: Option A ($20$) omits the multiplication by $s$. Option C ($80$) doubles the answer. Option D ($8$) uses $s^2 \cdot \frac{ds}{dt}$ without the height factor.

Answer: It is incorrect; the correct differentiation gives $\frac{x\frac{dx}{dt} + y\frac{dy}{dt}}{\sqrt{x^2 + y^2}}$

Correct differentiation: Using the chain rule: $\frac{d}{dt}\sqrt{x^2+y^2} = \frac{1}{2\sqrt{x^2+y^2}} \cdot \left(2x\frac{dx}{dt}+2y\frac{dy}{dt}\right) = \frac{x\frac{dx}{dt}+y\frac{dy}{dt}}{\sqrt{x^2+y^2}}$. Why the student's formula is wrong: The student confused the derivative of $\sqrt{x^2+y^2}$ with $\sqrt{(dx/dt)^2+(dy/dt)^2}$. The latter is the speed (magnitude of the velocity vector) of a point moving in 2D, which is a different quantity. Why distractors fail: Option A: $\frac{d}{dt}[\text{distance}]$ is not generally the same as speed — it equals speed only if the distance is measured from the origin along the trajectory. Option B: the chain rule does not produce the student's expression. Option D: the expression is certainly differentiable.

Answer: $40\pi$ m²/s

Write the area formula and differentiate: $A = \pi r^2$. Differentiating with respect to $t$: $\frac{dA}{dt} = 2\pi r \frac{dr}{dt}$. Substitute the given values: With $r = 10$ m and $\frac{dr}{dt} = 2$ m/s: $\frac{dA}{dt} = 2\pi(10)(2) = 40\pi$ m²/s. Why distractors fail: Option A ($20\pi$) omits the factor of 2 from the derivative. Option C ($100\pi$) uses $\pi r^2 \frac{dr}{dt}$ instead of $2\pi r \frac{dr}{dt}$. Option D ($4\pi$) only multiplies $2\pi$ by $\frac{dr}{dt}$ without the radius.

Answer: Both approaches are valid and yield the same answer

Analyze Approach A: Substituting $r = \frac{h}{3}$ into $V = \frac{1}{3}\pi r^2 h$ gives $V = \frac{\pi h^3}{27}$, then $\frac{dV}{dt} = \frac{\pi h^2}{9}\frac{dh}{dt}$. Analyze Approach B: Differentiating $V = \frac{1}{3}\pi r^2 h$ using the product rule: $\frac{dV}{dt} = \frac{1}{3}\pi(2rh\frac{dr}{dt} + r^2\frac{dh}{dt})$. Substituting $r = \frac{h}{3}$ and $\frac{dr}{dt} = \frac{1}{3}\frac{dh}{dt}$: $\frac{dV}{dt} = \frac{1}{3}\pi\left(2\cdot\frac{h}{3}\cdot h \cdot \frac{1}{3}\frac{dh}{dt} + \frac{h^2}{9}\frac{dh}{dt}\right) = \frac{1}{3}\pi\left(\frac{2h^2}{9}+\frac{h^2}{9}\right)\frac{dh}{dt} = \frac{\pi h^2}{9}\frac{dh}{dt}$. Why distractors fail: Options A and B each claim one approach is invalid, but both yield identical results. Option D is baseless since the product rule is applied correctly in Approach B.

Answer: Express the cross-sectional area as a function of depth $h$ using the equilateral triangle geometry, then write $V = 3A(h)$, differentiate, and solve for $\frac{dh}{dt}$

Plan the solution: For a trough with uniform cross section, $V = (\text{length}) \times A(h)$, where $A(h)$ is the cross-sectional area of water at depth $h$. The equilateral triangle geometry determines how the width of the water surface depends on $h$. Why Option B is optimal: This approach correctly identifies the need to: (1) express the cross-sectional area as a function of $h$ using the triangle's geometry, (2) multiply by the trough length to get $V(h)$, (3) differentiate with respect to $t$, and (4) solve for $\frac{dh}{dt}$. Why distractors fail: Option A doesn't account for varying water level. Option C uses a cone formula that doesn't apply to a prism. Option D is technically possible but less standard and more error-prone; it also doesn't outline the geometric setup.

Answer: $\frac{15}{11}$ m/s

Set up with similar triangles: Let $x$ be the person's distance from the post and $s$ be the length of the shadow. By similar triangles: $\frac{15}{x + s} = \frac{1.8}{s}$. Cross-multiplying: $15s = 1.8(x + s)$, so $15s = 1.8x + 1.8s$, giving $13.2s = 1.8x$, hence $s = \frac{3}{22}x$. Find the rate of the shadow tip: The shadow tip is at distance $x + s = x + \frac{3}{22}x = \frac{25}{22}x$ from the post. Differentiating: $\frac{d(x+s)}{dt} = \frac{25}{22}\frac{dx}{dt} = \frac{25}{22}(1.2) = \frac{30}{22} = \frac{15}{11} \approx 1.36$ m/s. Why distractors fail: Option B ($\frac{3}{22}$ m/s) confuses the shadow-length ratio $s = \frac{3}{22}x$ with the rate of the shadow tip. Option C ($2.4$ m/s) doubles the walking speed, a common error. Option D ($1.2$ m/s) only gives the person's walking speed, ignoring the shadow extension.

Answer: $\tan\theta = \frac{h}{100}$, where $h$ is the height of the balloon

Identify the triangle: The observer, the base of the balloon, and the balloon form a right triangle. The horizontal distance (adjacent) is $100$ m and the height (opposite) is $h$. Choose the correct trigonometric function: $\tan\theta = \frac{\text{opposite}}{\text{adjacent}} = \frac{h}{100}$. This is Option B. Why distractors fail: Option A uses $\sin\theta$ which needs the hypotenuse. Option C uses $\cos\theta$ with the wrong ratio. Option D mixes units and concepts — $\theta$ is an angle, not a length.

Answer: $450$ dollars/week

Differentiate the cost function with respect to time: $\frac{dC}{dt} = (5 + 0.02q)\frac{dq}{dt}$. Substitute values: At $q = 200$ with $\frac{dq}{dt} = 50$: $\frac{dC}{dt} = (5 + 0.02 \times 200)(50) = (5 + 4)(50) = 9(50) = 450$ dollars/week. Why distractors fail: Option A ($250$) uses only the linear term $5 \times 50$. Option C ($200$) uses only the quadratic term $0.02(200)(50)$. Option D ($500$) miscalculates $0.02 \times 200 = 5$ instead of $4$.

Answer: $\frac{1}{2\pi}$ cm/s

Differentiate the volume formula: $V = \pi h^2\left(R - \frac{h}{3}\right) = \pi\left(Rh^2 - \frac{h^3}{3}\right)$. With $R = 6$: $V = \pi(6h^2 - \frac{h^3}{3})$. $\frac{dV}{dt} = \pi(12h - h^2)\frac{dh}{dt}$. Substitute and solve: At $h = 2$: $\frac{dV}{dt} = \pi(24 - 4)\frac{dh}{dt} = 20\pi\frac{dh}{dt}$. Setting $10 = 20\pi\frac{dh}{dt}$ gives $\frac{dh}{dt} = \frac{10}{20\pi} = \frac{1}{2\pi}$ cm/s. Why distractors fail: Option A ($\frac{5}{12\pi}$) uses $12h = 24$ in the denominator without subtracting $h^2$. Option B ($\frac{5}{32\pi}$) incorrectly computes $12h - h^2$ at $h = 2$ as $32$. Option D ($\frac{1}{4\pi}$) uses $\frac{dV}{dt} = 10$ with an incorrect derivative coefficient of $40\pi$.

Answer: $12$

Differentiate the curve equation: $y = x^2 \Rightarrow \frac{dy}{dt} = 2x\frac{dx}{dt}$. Substitute values: At $x = 3$ with $\frac{dx}{dt} = 2$: $\frac{dy}{dt} = 2(3)(2) = 12$. Why distractors fail: Option A ($6$) computes $2x$ but forgets $\frac{dx}{dt}$. Option B ($9$) uses $x^2$ instead of $2x\frac{dx}{dt}$. Option D ($18$) uses $3x$ instead of $2x$.

Answer: Yes, and it gives the same result as implicit differentiation: $\frac{dr}{dt} = 1$ cm/s

Check both approaches: Implicit: $\frac{dA}{dt} = 2\pi r\frac{dr}{dt} \Rightarrow \frac{dr}{dt} = \frac{6\pi}{2\pi(3)} = 1$ cm/s. Explicit: $\frac{dr}{dt} = \frac{1}{2\sqrt{\pi A}}\cdot 6\pi$. At $r=3$, $A = 9\pi$, so $\frac{dr}{dt} = \frac{6\pi}{2\sqrt{9\pi^2}} = \frac{6\pi}{6\pi} = 1$ cm/s. Conclusion: Both methods are valid and yield the same answer. The student's approach is unconventional but mathematically correct. Why distractors fail: Option A incorrectly claims implicit differentiation is mandatory. Option B: the formula is correct. Option D: both approaches give the same answer.

Answer: It ensures the problem has a unique solution by reducing the number of unknown rates

Purpose of the geometric constraint: The volume formula $V = \frac{1}{3}\pi r^2 h$ involves two unknowns ($r$ and $h$). Substituting $r = \frac{h}{3}$ eliminates $r$, expressing $V$ in terms of $h$ alone: $V = \frac{1}{3}\pi \left(\frac{h}{3}\right)^2 h = \frac{\pi h^3}{27}$. Why the correct answer works: Option A is correct: by reducing the number of independent variables from two to one, we can solve for the single unknown rate $\frac{dh}{dt}$ given $\frac{dV}{dt}$. Why distractors fail: Option B: the chain rule works with multiple variables; having one variable is convenient, not required. Option C: the volume formula is valid regardless of the constraint. Option D: the derivative does not become zero; it simplifies the expression.

Answer: $4\pi r^2 \frac{dr}{dt}$

Differentiate using the chain rule: Since $r$ is a function of $t$, we differentiate $V = \frac{4}{3}\pi r^3$ with respect to $t$: $\frac{dV}{dt} = \frac{4}{3}\pi \cdot 3r^2 \cdot \frac{dr}{dt} = 4\pi r^2 \frac{dr}{dt}$. Why the correct answer works: Option B correctly applies the chain rule, bringing down the exponent 3, multiplying by $r^2$, and including the factor $\frac{dr}{dt}$. Why distractors fail: Option A omits the $\frac{dr}{dt}$ factor (it treats $r$ as a constant). Option C inverts the derivative as $\frac{dt}{dr}$. Option D has an incorrect exponent of $r^3$ instead of $r^2$.

Answer: $\frac{9}{5}$ m/s

Set up the relationship: Let $x$ be the horizontal distance and $L$ the string length. The height is constant at $40$ m. Then $L^2 = x^2 + 40^2 = x^2 + 1600$. Differentiate and solve: $2L\frac{dL}{dt} = 2x\frac{dx}{dt}$. When $x = 30$: $L = \sqrt{900 + 1600} = 50$. So $50\frac{dL}{dt} = 30(3) = 90$, giving $\frac{dL}{dt} = \frac{90}{50} = \frac{9}{5}$ m/s. Why distractors fail: Option B assumes the string extends at the same rate the kite moves horizontally. Option C incorrectly uses $L = \sqrt{30^2 + 40^2}$ but miscomputes. Option D divides by $L$ twice.

Answer: The approach is incorrect; the product rule gives $\frac{dA}{dt} = \frac{1}{2}\left(b\frac{dh}{dt} + h\frac{db}{dt}\right)$

Apply the product rule: Since both $b$ and $h$ depend on $t$, $\frac{dA}{dt} = \frac{1}{2}\left(\frac{db}{dt} \cdot h + b \cdot \frac{dh}{dt}\right)$ by the product rule. Identify the student's error: The student treated $b$ as a constant when differentiating, which is only valid if $b$ does not change. The problem states both change. Why distractors fail: Option A is only valid if $b$ is constant, which it is not. Option B multiplies the two rates together, which is not the product rule. Option D is absurd — of course the area can change.

Answer: $\frac{dV}{dt}$ increases proportionally to $s^2$

Differentiate the cube volume: $V = s^3 \Rightarrow \frac{dV}{dt} = 3s^2 \frac{ds}{dt} = 3s^2(2) = 6s^2$. Analyze the dependence: Since $\frac{dV}{dt} = 6s^2$, the rate at which volume changes grows in proportion to $s^2$. As $s$ gets larger, the volume changes more rapidly. Why distractors fail: Option A is incorrect because even though $\frac{ds}{dt}$ is constant, the factor $s^2$ grows. Option C confuses $V = s^3$ with $\frac{dV}{dt}$. Option D contradicts the formula.

Answer: The rate of change of volume with respect to time

Interpret the Leibniz notation: $\frac{dV}{dt}$ uses Leibniz notation where the numerator indicates the quantity being differentiated ($V$, volume) and the denominator indicates the variable of differentiation ($t$, time). Why the correct answer works: Option B correctly identifies $\frac{dV}{dt}$ as the instantaneous rate of change of volume with respect to time — this is the fundamental meaning of a derivative. Why distractors fail: Option A confuses $V(t)$ (the function) with its derivative. Option C reverses the roles of numerator and denominator. Option D describes a definite integral $\int V\,dt$, not a derivative.

Answer: $0.03$ rad/s

Set up the relationship: Let $x$ be the horizontal distance from the observer to the plane. Then $\tan\theta = \frac{3000}{x}$, so $\theta = \arctan\frac{3000}{x}$. Differentiate: Differentiating $\tan\theta = \frac{3000}{x}$: $\sec^2\theta \cdot \frac{d\theta}{dt} = -\frac{3000}{x^2}\frac{dx}{dt}$. When $x = 4000$: $\sec^2\theta = 1 + \tan^2\theta = 1 + \left(\frac{3}{4}\right)^2 = \frac{25}{16}$. So $\frac{25}{16}\frac{d\theta}{dt} = -\frac{3000}{16000000}(250) = -\frac{750000}{16000000} = -\frac{3}{64}$. Solve: $\frac{d\theta}{dt} = -\frac{3}{64} \cdot \frac{16}{25} = -\frac{48}{1600} = -0.03$ rad/s. The magnitude is $0.03$ rad/s. Why distractors fail: Options B, C, and D result from common errors such as omitting $\sec^2\theta$, using incorrect values of $x^2$, or miscomputing the trigonometric factor.

Answer: $\frac{-1}{3\pi}$ cm/min

Differentiate the volume formula: $V = \frac{4}{3}\pi r^3 \Rightarrow \frac{dV}{dt} = 4\pi r^2 \frac{dr}{dt}$. Solve for $\frac{dr}{dt}$: $\frac{dr}{dt} = \frac{\frac{dV}{dt}}{4\pi r^2} = \frac{-12}{4\pi(9)} = \frac{-12}{36\pi} = \frac{-1}{3\pi}$ cm/min. Why distractors fail: Option B ($\frac{-1}{\pi}$) neglects the factor of $r^2 = 9$ in the denominator correctly but divides $-12$ by $4\pi$ only, omitting $r^2$. Wait — $-12/(4\pi) = -3/\pi$, so this distractor uses $-12/(12\pi)$ which misapplies the formula. Option C ($\frac{-4}{3\pi}$) uses $r$ instead of $r^2$ in the denominator. Option D ($\frac{-1}{9\pi}$) incorrectly uses $r^2 = 9$ twice or divides by an extra factor of 3.

Answer: $\frac{5}{4\pi}$ m/min

Use the geometric constraint: The ratio $\frac{r}{h} = \frac{4}{12} = \frac{1}{3}$, so $r = \frac{h}{3}$. Substituting into $V = \frac{1}{3}\pi r^2 h$ gives $V = \frac{1}{3}\pi\left(\frac{h}{3}\right)^2 h = \frac{\pi h^3}{27}$. Differentiate and solve: $\frac{dV}{dt} = \frac{\pi}{27} \cdot 3h^2 \frac{dh}{dt} = \frac{\pi h^2}{9}\frac{dh}{dt}$. At $h = 6$: $5 = \frac{\pi(36)}{9}\frac{dh}{dt} = 4\pi \frac{dh}{dt}$, so $\frac{dh}{dt} = \frac{5}{4\pi}$ m/min. Why distractors fail: Option B omits the factor from the geometric constraint. Option C uses $h^2 = 36$ with an extra factor of 4 in the denominator. Option D multiplies instead of divides at the final step.

Answer: $2x\frac{dx}{dt} + 2y\frac{dy}{dt} = 0$

Apply the chain rule to each term: Since $x = x(t)$ and $y = y(t)$, $\frac{d}{dt}[x^2] = 2x\frac{dx}{dt}$ and $\frac{d}{dt}[y^2] = 2y\frac{dy}{dt}$ by the chain rule. The right side $L^2$ is constant, so its derivative is $0$. Why the correct answer works: Option B correctly applies the chain rule to both $x^2$ and $y^2$ and recognizes $L$ is constant. Why distractors fail: Option A omits the $\frac{dx}{dt}$ and $\frac{dy}{dt}$ factors entirely. Option C incorrectly keeps the squared terms in front of the rates. Option D treats $L$ as a variable, but the problem states $L$ is constant.

Answer: $2400\pi$ m/min

Determine $\frac{d\theta}{dt}$: The light makes 3 revolutions per minute, so $\frac{d\theta}{dt} = 3 \times 2\pi = 6\pi$ rad/min. Set up and differentiate: Let $x$ be the distance along the shore from $P$. Then $\tan\theta = \frac{x}{200}$, so $\frac{dx}{dt} = 200\sec^2\theta \cdot \frac{d\theta}{dt}$. When $x = 200$: $\tan\theta = 1$, so $\theta = \frac{\pi}{4}$ and $\sec^2\theta = 2$. Compute the answer: $\frac{dx}{dt} = 200(2)(6\pi) = 2400\pi$ m/min. Why distractors fail: Option B uses $\sec^2\theta = 1$. Option C uses $\frac{d\theta}{dt} = 3\pi$ instead of $6\pi$. Option D doubles the correct answer.

Answer: Identify all changing quantities and the equation that relates them

The standard procedure: The first step in any related rates problem is to identify which quantities are changing, which rates are given, which rate is sought, and find an equation relating the changing quantities. Why the correct answer works: Option C describes this essential setup step: identify all variables and the connecting equation before any differentiation. Why distractors fail: Option A skips the crucial setup. Option B is a common error — you should only substitute constant values before differentiating; changing quantities must remain as variables. Option D is not always possible or necessary; implicit differentiation avoids needing to isolate a variable.

Answer: $-\frac{8}{3 \cdot 9^{2/3}}$

Find $y$ when $x = 1$: $1 + y^3 = 10 \Rightarrow y^3 = 9 \Rightarrow y = 9^{1/3} = \sqrt[3]{9} \approx 2.08$. Differentiate implicitly: $2x\frac{dx}{dt} + 3y^2\frac{dy}{dt} = 0 \Rightarrow \frac{dy}{dt} = -\frac{2x\frac{dx}{dt}}{3y^2} = -\frac{2(1)(4)}{3(9^{1/3})^2} = -\frac{8}{3 \cdot 9^{2/3}}$. Why distractors fail: Option A ($-\frac{8}{27}$) incorrectly assumes $y = 3$ (i.e., $y^3 = 27$ instead of $9$). Option B ($-\frac{2}{3}$) results from setting $y = 2$ in the denominator. Option C ($-\frac{4}{9}$) uses $3y^2 = 3(3)^2 = 27$ again assuming $y = 3$, then simplifies incorrectly.

Answer: $175\pi$ cm³/s

Differentiate the cylinder volume: $V = \pi r^2 h$. Using the product rule: $\frac{dV}{dt} = \pi\left(2rh\frac{dr}{dt} + r^2\frac{dh}{dt}\right)$. Substitute values: $\frac{dV}{dt} = \pi\left(2(5)(10)(1) + (25)(3)\right) = \pi(100 + 75) = 175\pi$ cm³/s. Why distractors fail: Option B ($100\pi$) only includes the $2rh\frac{dr}{dt}$ term. Option C ($75\pi$) only includes the $r^2\frac{dh}{dt}$ term. Option D ($150\pi$) miscomputes one of the terms.

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

Find the net rate of volume change: The net rate is $\frac{dV}{dt} = 3 - 1 = 2$ m³/min. Relate volume to height: For a cylinder: $V = \pi r^2 h = 4\pi h$. Differentiating: $\frac{dV}{dt} = 4\pi \frac{dh}{dt}$, so $\frac{dh}{dt} = \frac{2}{4\pi} = \frac{1}{2\pi}$ m/min. Why distractors fail: Option A uses $\frac{dV}{dt} = 3$ (ignoring drainage). Option C uses $\frac{dV}{dt} = 1$. Option D uses $r = 1$ instead of $r = 2$.

Answer: The sign should be negative since the radius is decreasing

Sign conventions in related rates: In related rates, a decreasing quantity has a negative rate of change. If the radius is shrinking, $\frac{dr}{dt} < 0$. Why the correct answer works: Option B correctly identifies that a positive $\frac{dr}{dt}$ contradicts the statement that the circle is shrinking. Why distractors fail: Option A makes no valid mathematical claim. Option C is wrong because $\frac{dr}{dt}$ has units of length per time, not area per time. Option D ignores the sign convention that is fundamental to related rates.

Answer: They replaced a changing variable with a constant before differentiating, making its derivative zero

Identify the error: If $r = 3$ is substituted before differentiation, $V$ becomes $\frac{4}{3}\pi(27) = 36\pi$, a constant. Differentiating a constant gives $0$. The correct procedure: Quantities that change with time must remain as variables during differentiation. Substitute their specific values only after differentiating. Why distractors fail: Option A: the formula is correct. Option B: there is no product of two functions of $t$ here. Option D: in related rates, we differentiate with respect to $t$, not $r$.

Answer: $\frac{50\pi}{3}$ m/s

Set up the trigonometric relationship: Let $x$ be the distance along the wall from the nearest point. Then $\tan\theta = \frac{x}{50}$, so $x = 50\tan\theta$. Differentiate with respect to $t$: $\frac{dx}{dt} = 50\sec^2\theta \cdot \frac{d\theta}{dt}$. At $\theta = \frac{\pi}{4}$: $\sec^2\frac{\pi}{4} = 2$. So $\frac{dx}{dt} = 50(2)\left(\frac{\pi}{6}\right) = \frac{100\pi}{6} = \frac{50\pi}{3}$ m/s. Why distractors fail: Option A ($\frac{50\pi}{6}$) uses $\sec^2\theta = 1$ (i.e., forgets the $\sec^2$ factor). Option C ($\frac{25\pi}{3}$) uses $25$ instead of $50$. Option D ($\frac{25\pi}{6}$) makes both errors: using $25$ and omitting $\sec^2\theta$.

Answer: $x^2 + y^2 = 25$

Identify the geometric relationship: The wall, the ground, and the ladder form a right triangle. The ladder is the hypotenuse with constant length 5 m. Apply the Pythagorean theorem: By the Pythagorean theorem: $x^2 + y^2 = 5^2 = 25$. Why distractors fail: Option A uses a linear relationship which does not apply to right triangles. Option C has the wrong sign. Option D uses a product relationship with no geometric basis here.

Answer: The claim is incorrect because $\frac{dy}{dt}$ depends on $x$ and $y$, which both change over time

Examine the rate equation: From $x^2 + y^2 = L^2$, we get $\frac{dy}{dt} = -\frac{x}{y}\frac{dx}{dt}$. Even if $\frac{dx}{dt}$ is constant, the ratio $\frac{x}{y}$ changes as the ladder slides, so $\frac{dy}{dt}$ changes over time. Why the correct answer works: Option C correctly identifies that the rate at which the top descends depends on the ratio $\frac{x}{y}$, which varies. As $y \to 0$, the rate $|\frac{dy}{dt}| \to \infty$. Why distractors fail: Option A: constant length does not imply constant rates for both endpoints. Option B: the Pythagorean relationship $x^2 + y^2 = L^2$ is not linear. Option D: gravity is irrelevant in the mathematical setup of the problem.

Answer: The rate of change of the distance equals $\sqrt{60^2 + 80^2} = 100$ km/h and remains constant

Set up the distance equation: Let $x = 80t$ (east) and $y = 60t$ (north). The distance $D = \sqrt{x^2 + y^2}$. Then $D^2 = x^2 + y^2$, so $2D\frac{dD}{dt} = 2x\frac{dx}{dt} + 2y\frac{dy}{dt}$. Compute the rate: Substituting: $D\frac{dD}{dt} = (80t)(80) + (60t)(60) = t(6400 + 3600) = 10000t$. Since $D = \sqrt{6400t^2 + 3600t^2} = 100t$, we get $100t \cdot \frac{dD}{dt} = 10000t$, so $\frac{dD}{dt} = 100$ km/h (constant). Why distractors fail: Option A incorrectly adds speeds linearly. Option C is false since 100 > 80 and 100 > 60. Option D is incorrect because the rate is actually constant at 100 km/h in this perpendicular-departure scenario.

Answer: $\frac{26}{\sqrt{153}}$ m/s

Set up the relationship: Let $x$ be the horizontal distance from the boat to the dock and $L$ be the length of rope. By the Pythagorean theorem: $x^2 + 4^2 = L^2$, so $x^2 + 16 = L^2$. Differentiate and solve: $2x\frac{dx}{dt} = 2L\frac{dL}{dt}$, so $\frac{dx}{dt} = \frac{L}{x}\frac{dL}{dt}$. When $L = 13$: $x = \sqrt{169 - 16} = \sqrt{153}$. With $\frac{dL}{dt} = -2$: $\frac{dx}{dt} = \frac{13}{\sqrt{153}}(-2) = -\frac{26}{\sqrt{153}}$. The boat approaches at $\frac{26}{\sqrt{153}}$ m/s. Why distractors fail: Option B assumes the boat moves at the same speed as the rope is pulled. Option C uses $x = 12$ (from a 5-12-13 triangle, but the height is 4, not 5). Option D omits the factor of 2 from $\frac{dL}{dt}$.

Answer: $-\frac{3}{4}$ m/s

Set up the Pythagorean relationship: $x^2 + y^2 = 100$ where $x$ is the distance of the base from the wall and $y$ is the height of the top. Differentiate and solve: $2x\frac{dx}{dt} + 2y\frac{dy}{dt} = 0$. When $x = 6$, $y = \sqrt{100-36} = 8$. With $\frac{dx}{dt} = 1$: $2(6)(1) + 2(8)\frac{dy}{dt} = 0 \Rightarrow \frac{dy}{dt} = -\frac{12}{16} = -\frac{3}{4}$ m/s. Why distractors fail: Option B ($-\frac{4}{3}$) inverts the fraction $\frac{x}{y}$. Option C ($-\frac{1}{2}$) results from using $y = 12$ or other algebraic errors. Option D ($-\frac{8}{6} = -\frac{4}{3}$) reverses the $x$ and $y$ values in the ratio.

Answer: Implicit differentiation with respect to time $t$

Core technique of related rates: Related rates problems involve multiple quantities that change over time. Because the relationship between these quantities is typically not solved for a single variable explicitly as a function of $t$, we differentiate both sides of the equation implicitly with respect to $t$. Why the correct answer works: Option B is correct because implicit differentiation with respect to time, applying the chain rule to each variable, is the defining technique. Why distractors fail: Option A involves differentiating $y$ with respect to $x$, not $t$. Options C and D are integration techniques, not differentiation techniques.