Differentiating Inverse Functions

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AP Calculus BC › Differentiating Inverse Functions

Questions 1 - 10
1

Let $f(x)=x^3-2x+5$ be invertible and $f(1)=4$; what is $(f^{-1})'(4)$?

$\dfrac{1}{f'(1)}$

$f'(1)$

$f'(4)$

$\dfrac{1}{3}$

$\dfrac{1}{f'(4)}$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point b, use the formula $(f^{-1}$)'(b) = 1 / f'(a), where a is the value such that f(a) = b. In this case, f(1) = 4, so a = 1 and b = 4. Therefore, $(f^{-1}$)'(4) = 1 / f'(1). A tempting distractor is choice B, 1/f'(4), which incorrectly uses the derivative at b instead of at a. A transferable strategy for inverse derivatives is to identify the point a where f(a) = b and then compute the reciprocal of f'(a).

2

For $f(x)=x^5$ (invertible) with $f(2)=32$, what is $(f^{-1})'(32)$?

$\dfrac{1}{f'(2)}$

$\dfrac{1}{f'(32)}$

$f'(2)$

$\dfrac{1}{80}$

$80$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point b, use the formula $ (f^{-1})'(b) = \dfrac{1}{f'(a)} $, where a is the value such that $ f(a) = b $. In this case, $ f(2) = 32 $, so a = 2 and b = 32. Therefore, $ (f^{-1})'(32) = \dfrac{1}{f'(2)} $. A tempting distractor is choice B, $ \dfrac{1}{f'(32)} $, which incorrectly uses the derivative at b instead of at a. A transferable strategy for inverse derivatives is to identify the point a where $ f(a) = b $ and then compute the reciprocal of $ f'(a) $.

3

Let $f(x)=e^x+2$ be invertible and $f(0)=3$; what is $(f^{-1})'(3)$?

$\dfrac{1}{f'(0)}$

$e^3$

$f'(0)$

$\dfrac{1}{f'(3)}$

$\dfrac{1}{e^3}$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point $b$, use the formula $(f^{-1})'(b) = \frac{1}{f'(a)}$, where $a$ is the value such that $f(a) = b$. In this case, $f(0) = 3$, so $a = 0$ and $b = 3$. Therefore, $(f^{-1})'(3) = \frac{1}{f'(0)}$. A tempting distractor is choice E, $\frac{1}{f'(3)}$, which incorrectly uses the derivative at $b$ instead of at $a$. A transferable strategy for inverse derivatives is to identify the point $a$ where $f(a) = b$ and then compute the reciprocal of $f'(a)$.

4

Given $f(x)=\dfrac{x-1}{x+1}$ (invertible on $x>-1$) and $f(1)=0$, find $(f^{-1})'(0)$.

$f'(0)$

$\dfrac{1}{f'(0)}$

$\dfrac{1}{f'(1)}$

$f'(1)$

$\dfrac{1}{2}$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point b, use the formula $(f^{-1}$)'(b) = 1 / f'(a), where a is the value such that f(a) = b. Here, f(1) = 0, so a = 1 and b = 0. Thus, $(f^{-1}$)'(0) = 1 / f'(1). A tempting distractor is choice A, 1/f'(0), which mistakenly evaluates the derivative at b rather than at a. A transferable strategy for inverse derivatives is to identify the point a where f(a) = b and then compute the reciprocal of f'(a).

5

If $f(x)=\ln(x-1)$ for $x>1$ and $f(e+1)=1$, what is $(f^{-1})'(1)$?

$\dfrac{1}{f'(1)}$

$e+1$

$\dfrac{1}{e}$

$\dfrac{1}{f'(e+1)}$

$f'(e+1)$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point b, use the formula $(f^{-1})'(b) = \frac{1}{f'(a)}$, where a is the value such that $f(a) = b$. Here, $f(e+1) = 1$, so a = e+1 and b = 1. Thus, $(f^{-1})'(1) = \frac{1}{f'(e+1)}$. A tempting distractor is choice A, $1/f'(1)$, which mistakenly evaluates the derivative at b rather than at a. A transferable strategy for inverse derivatives is to identify the point a where $f(a) = b$ and then compute the reciprocal of $f'(a)$.

6

If $f(x)=\sqrt{x+1}$ for $x\ge -1$ and $f(3)=2$, what is $(f^{-1})'(2)$?

$\dfrac{1}{f'(3)}$

$\dfrac{1}{f'(2)}$

$f'(3)$

$2$

$\dfrac{1}{4}$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point b, use the formula $(f^{-1}$)'(b) = 1 / f'(a), where a is the value such that f(a) = b. In this case, f(3) = 2, so a = 3 and b = 2. Therefore, $(f^{-1}$)'(2) = 1 / f'(3). A tempting distractor is choice B, 1/f'(2), which incorrectly uses the derivative at b instead of at a. A transferable strategy for inverse derivatives is to identify the point a where f(a) = b and then compute the reciprocal of f'(a).

7

Let $f(x)=x^3+1$ be invertible and $f(0)=1$; what is $(f^{-1})'(1)$?

$\dfrac{1}{f'(0)}$

$\dfrac{1}{f'(1)}$

$f'(0)$

$f'(1)$

$\dfrac{1}{3}$

Explanation

This problem tests the skill of differentiating inverse functions. To find the derivative of the inverse function at a point b, use the formula $(f^{-1})'(b) = 1 / f'(a)$, where a is the value such that $f(a) = b$. In this case, $f(0) = 1$, so a = 0 and b = 1. Therefore, $(f^{-1})'(1) = 1 / f'(0)$. A tempting distractor is choice A, $1/f'(1)$, which incorrectly uses the derivative at b instead of at a. A transferable strategy for inverse derivatives is to identify the point a where $f(a) = b$ and then compute the reciprocal of $f'(a)$.

8

Let $f(x)=\sqrt{x+1}$ for $x\ge-1$ and $f(3)=2$. What is $(f^{-1})'(2)$?

$4$

$\dfrac{1}{4}$

$\dfrac{1}{8}$

$\dfrac{1}{2}$

$2$

Explanation

To find the derivative of an inverse function, we apply $(f^{-1})'(b) = rac{1}{f'(a)}$ where $f(a) = b$. Given $f(3) = 2$, we need $(f^{-1})'(2) = rac{1}{f'(3)}$. For $f(x) = sqrt{x+1}$, we have $f'(x) = rac{1}{2sqrt{x+1}}$. Evaluating at $x = 3$: $f'(3) = rac{1}{2sqrt{4}} = rac{1}{4}$. Therefore, $(f^{-1})'(2) = 4$. Students might forget the chain rule when differentiating $sqrt{x+1}$ or evaluate at the wrong point. The key is recognizing that inverse derivatives reciprocate the original function's rate of change.

9

Let $f(x)=\sin x+x$ on $-\pi/2,\pi/2$ with $f(0)=0$. Find $(f^{-1})'(0)$.

$0$

$1$

$-\dfrac{1}{2}$

$\dfrac{1}{2}$

$2$

Explanation

This problem requires finding the derivative of an inverse function at a point. Using $(f^{-1})'(b) = rac{1}{f'(a)}$ where $f(a) = b$, and knowing $f(0) = 0$, we need $(f^{-1})'(0) = rac{1}{f'(0)}$. Taking the derivative: $f'(x) = cos x + 1$, we get $f'(0) = cos(0) + 1 = 1 + 1 = 2$. Thus $(f^{-1})'(0) = rac{1}{2}$. A common mistake is computing $f'$ at the wrong point or forgetting that $cos(0) = 1$, not 0. Remember that the inverse function derivative formula creates a reciprocal relationship between the rates of change.

10

Let $f(x)=e^x+x$ with $f(0)=1$. What is $(f^{-1})'(1)$?

$e$

$2$

$\dfrac{1}{2}$

$\dfrac{1}{e}$

$1$

Explanation

To find the derivative of an inverse function, we use the relationship $(f^{-1})'(b) = rac{1}{f'(a)}$ where $f(a) = b$. Given $f(0) = 1$, we need $(f^{-1})'(1) = rac{1}{f'(0)}$. Taking the derivative, $f'(x) = e^x + 1$, so $f'(0) = e^0 + 1 = 2$. Therefore, $(f^{-1})'(1) = rac{1}{2}$. Students might mistakenly compute $f'(1) = e + 1$ by plugging in the wrong value, which would give approximately $ rac{1}{3.718}$. Remember to use the input value that produces your target output, not the output value itself.

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