Defining and Differentiating Parametric Equations

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AP Calculus BC › Defining and Differentiating Parametric Equations

Questions 1 - 10
1

Given $x(t)=e^{2t}$ and $y(t)=e^t+1$, compute $\frac{dy}{dx}$ in terms of $t$.

$\dfrac{e^t}{2e^{2t}}$

$\dfrac{2e^{2t}}{e^t}$

$(e^t)(2e^{2t})$

$\dfrac{e^t+1}{2e^{2t}}$

$\dfrac{e^t}{e^{2t}}$

Explanation

This parametric differentiation requires $\frac{dy}{dx} = \frac{\frac{dy}{dt}}{\frac{dx}{dt}}$. For $x(t) = e^{2t}$, we find $\frac{dx}{dt} = 2e^{2t}$. For $y(t) = e^t + 1$, we get $\frac{dy}{dt} = e^t$. The parametric derivative formula gives $\frac{dy}{dx} = \frac{e^t}{2e^{2t}} = \frac{1}{2e^t}$. Choice A shows the unreduced form. The reliable parametric derivative strategy is to differentiate each component separately, then form their proper quotient.

2

For $x(t)=\frac{t^2+1}{t}$ and $y(t)=\frac{t^2-1}{t}$, $t\ne0$, determine $\frac{dy}{dx}$.

$(1-1/t^2)(1+1/t^2)$

$\dfrac{1+1/t^2}{1-1/t^2}$

$\dfrac{(t^2-1)/t}{1-1/t^2}$

$\dfrac{1-1/t^2}{1+1/t^2}$

$\dfrac{1-1/t^2}{(t^2+1)/t}$

Explanation

This parametric differentiation applies dy/dx = (dy/dt)/(dx/dt). For x(t) = (t² + 1)/t, we can rewrite as t + 1/t, so dx/dt = 1 - 1/t². For y(t) = (t² - 1)/t = t - 1/t, we get dy/dt = 1 + 1/t². The parametric derivative formula yields dy/dx = (1 + 1/t²)/(1 - 1/t²). Choice B incorrectly inverts the fraction. The standard parametric differentiation method involves finding derivatives of both components, then dividing dy/dt by dx/dt.

3

A parametric path is given by $x(t)=e^t$ and $y(t)=t e^t$; what is $\frac{dy}{dx}$ in terms of $t$?

$\dfrac{e^t+te^t}{e^t}$

$\dfrac{e^t+te^t}{t e^t}$

$\dfrac{e^t}{e^t+te^t}$

$\dfrac{te^t}{e^t}$

$(e^t)(e^t+te^t)$

Explanation

This parametric curve requires applying $ \frac{dy}{dx} = \frac{dy/dt}{dx/dt} $ to find the derivative. For $ x(t) = e^t $, we have $ dx/dt = e^t $. For $ y(t) = t e^t $, we use the product rule: $ dy/dt = e^t + t e^t $. The chain rule then gives $ dy/dx = (e^t + t e^t) / e^t $. Choice B incorrectly inverts this ratio by placing $ dx/dt $ in the numerator instead of the denominator. The essential parametric differentiation approach is to compute $ dy/dt $ and $ dx/dt $ separately, then divide the first by the second.

4

A curve is $x(t)=t^2+\frac{1}{2}t$ and $y(t)=t^2-\frac{1}{2}t$; find $\frac{dy}{dx}$.

$\dfrac{2t-1/2}{t^2+\tfrac12 t}$

$\dfrac{t^2-\tfrac12 t}{2t+1/2}$

$(2t-1/2)(2t+1/2)$

$\dfrac{2t-1/2}{2t+1/2}$

$\dfrac{2t+1/2}{2t-1/2}$

Explanation

This parametric curve applies dy/dx = (dy/dt)/(dx/dt). For x(t) = t² + t/2, we calculate dx/dt = 2t + 1/2. For y(t) = t² - t/2, we get dy/dt = 2t - 1/2. Using the chain rule yields dy/dx = (2t - 1/2)/(2t + 1/2). Choice B incorrectly inverts the fraction by putting dx/dt in the numerator. The essential parametric differentiation approach is to compute both component derivatives, then divide dy/dt by dx/dt.

5

For $x(t)=t\cos t$ and $y(t)=t\sin t$, determine $\frac{dy}{dx}$ in terms of $t$.

$\dfrac{t\sin t}{\cos t-t\sin t}$

$(\sin t+t\cos t)(\cos t-t\sin t)$

$\dfrac{\cos t-t\sin t}{\sin t+t\cos t}$

$\dfrac{\sin t+t\cos t}{\cos t-t\sin t}$

$\dfrac{\sin t+t\cos t}{t\cos t}$

Explanation

This parametric differentiation uses dy/dx = (dy/dt)/(dx/dt). For x(t) = t cos t, we apply the product rule: dx/dt = cos t - t sin t. For y(t) = t sin t, we get dy/dt = sin t + t cos t. The parametric derivative gives dy/dx = (sin t + t cos t)/(cos t - t sin t). Choice B incorrectly inverts the fraction. The key parametric derivative technique is to differentiate each component separately, then form the ratio with dy/dt in the numerator.

6

A particle moves with $x(t)=\frac{1}{2}t^2$ and $y(t)=\frac{1}{4}t^4$; compute $\frac{dy}{dx}$.

$\dfrac{t^3}{t^2/2}$

$(t^3)(t)$

$\dfrac{t}{t^3}$

$\dfrac{t^4/4}{t}$

$\dfrac{t^3}{t}$

Explanation

This parametric curve requires $dy/dx = (dy/dt)/(dx/dt)$. For $x(t) = \frac{1}{2}t^2$, we find $dx/dt = t$. For $y(t) = \frac{1}{4}t^4$, we get $dy/dt = t^3$. Using the parametric derivative formula yields $dy/dx = t^3/t = t^2$. Choice A shows this result correctly. The standard parametric differentiation method involves finding derivatives of both components, then dividing $dy/dt$ by $dx/dt$.

7

A curve is defined by $x(t)=t^3-2t$ and $y(t)=4t^2+t$; what is $\frac{dy}{dx}$?

$\dfrac{3t^2-2}{8t+1}$

$\dfrac{8t+1}{t^3-2t}$

$(8t+1)(3t^2-2)$

$\dfrac{4t^2+t}{3t^2-2}$

$\dfrac{8t+1}{3t^2-2}$

Explanation

This parametric curve requires the chain rule formula dy/dx = (dy/dt)/(dx/dt). For x(t) = t³ - 2t, we calculate dx/dt = 3t² - 2. For y(t) = 4t² + t, we get dy/dt = 8t + 1. Applying the parametric derivative rule yields dy/dx = (8t + 1)/(3t² - 2). Choice B incorrectly inverts the fraction by placing dx/dt in the numerator. The reliable parametric differentiation method is to compute both component derivatives, then divide dy/dt by dx/dt.

8

A parametric curve has $x(t)=t^2+\tan t$ and $y(t)=t^2+\sec t$; find $\frac{dy}{dx}$.

$\dfrac{2t+\sec t\tan t}{2t+\sec^2 t}$

$\dfrac{2t+\sec t\tan t}{t^2+\tan t}$

$\dfrac{t^2+\sec t}{2t+\sec^2 t}$

$(2t+\sec t\tan t)(2t+\sec^2 t)$

$\dfrac{2t+\sec^2 t}{2t+\sec t\tan t}$

Explanation

This parametric curve uses $\frac{dy}{dx} = \frac{\frac{dy}{dt}}{\frac{dx}{dt}}$. For $x(t) = t^2 + \tan t$, we calculate $\frac{dx}{dt} = 2t + \sec^2 t$. For $y(t) = t^2 + \sec t$, we get $\frac{dy}{dt} = 2t + \sec t \tan t$. Using the parametric derivative gives $\frac{dy}{dx} = \frac{2t + \sec t \tan t}{2t + \sec^2 t}$. Choice B incorrectly inverts the fraction. The fundamental parametric differentiation technique is to compute derivatives of both components, then divide $\frac{dy}{dt}$ by $\frac{dx}{dt}$.

9

A curve is given by $x(t)=t^3$ and $y(t)=\dfrac{1}{t}$ for $t\ne0$; what is $\dfrac{dy}{dx}$?

$-\dfrac{1}{3t^4}$

$\dfrac{3t^2}{-1/t^2}$

$3t^2$

$\dfrac{-1/t^2}{3t^2}$

$-\dfrac{1}{t^2}$

Explanation

This problem involves parametric differentiation to find $dy/dx$ for the given functions $x(t)$ and $y(t)$. To find $dy/dx$, we apply the chain rule, which states that $dy/dx = (dy/dt) / (dx/dt)$, assuming $dx/dt$ is not zero. First, compute $dx/dt = 3t^2$ from $x(t) = t^3$, and $dy/dt = -1/t^2$ from $y(t) = 1/t$. Therefore, $dy/dx = (-1/t^2) / (3t^2) = -1/(3t^4)$, which matches choice E. A tempting distractor is choice A, which is the unsimplified form $(-1/t^2)/(3t^2)$ but is actually equal to E, though the question likely expects the simplified version. In general, for parametric equations, always compute the ratio of the y-derivative to the x-derivative with respect to the parameter to obtain the correct slope.

10

Given $x(t)=\sin(2t)$ and $y(t)=\cos(3t)$, compute $\frac{dy}{dx}$ in terms of $t$.

$\dfrac{-3\sin(3t)}{2\cos(2t)}$

$\dfrac{\cos(3t)}{2\cos(2t)}$

$(-3\sin(3t))(2\cos(2t))$

$\dfrac{-\sin(3t)}{\cos(2t)}$

$\dfrac{2\cos(2t)}{-3\sin(3t)}$

Explanation

This parametric differentiation uses the chain rule dy/dx = (dy/dt)/(dx/dt). For x(t) = sin(2t), we find dx/dt = 2cos(2t). For y(t) = cos(3t), we get dy/dt = -3sin(3t). The parametric derivative formula gives dy/dx = (-3sin(3t))/(2cos(2t)). Choice B incorrectly inverts the fraction by placing dx/dt in the numerator. The key parametric differentiation strategy is to compute both component derivatives, then form the ratio dy/dt over dx/dt.

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