Differentiating Inverse Functions

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AP Calculus AB › Differentiating Inverse Functions

Questions 1 - 10
1

Given $g(x)=x^4+2$ and $g(1)=3$, what is $(g^{-1})'(3)$?

$4$

$\dfrac{1}{4}$

$g'(1)$

$\dfrac{1}{3}$

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

Explanation

This problem tests the skill of differentiating inverse functions. The derivative of the inverse function at a point a is given by 1 over the derivative of the original function at b, where g(b) = a. Here, g(1) = 3, so b = 1 and a = 3. Compute g'(x) = 4x³, so g'(1) = 4, thus (g⁻¹)'(3) = 1/4. A tempting distractor is A, 4, which might be chosen if one forgets to take the reciprocal, but that gives the slope of the tangent to g, not to g⁻¹. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

2

Let $f(x)=x^3+1$ and $f(2)=9$. What is $(f^{-1})'(9)$?

$\dfrac{1}{9}$

$12$

$\dfrac{1}{12}$

$\dfrac{1}{f'(9)}$

$f'(2)$

Explanation

This problem tests the skill of differentiating inverse functions. The derivative of the inverse function at a point a is given by 1 over the derivative of the original function at b, where f(b) = a. Here, f(2) = 9, so b = 2 and a = 9. Compute $f'(x) = 3x^2$, so f'(2) = 12, thus $(f^{-1})'(9) = \frac{1}{12}$. A tempting distractor is A, 12, which might be chosen if one forgets to take the reciprocal, but that gives the slope of the tangent to f, not to $f^{-1}$. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

3

Let $f(x)=x^3+2x-5$ and $f(2)=7$. What is $(f^{-1})'(7)$?

$\dfrac{1}{f'(7)}$

$f'(2)$

$\dfrac{1}{14}$

$\dfrac{1}{7}$

$14$

Explanation

This problem tests the skill of differentiating inverse functions. The derivative of the inverse function at a point a is given by 1 over the derivative of the original function at b, where f(b) = a. Here, f(2) = 7, so b = 2 and a = 7. Compute f'(x) = 3x² + 2, so f'(2) = 14, thus (f⁻¹)'(7) = 1/14. A tempting distractor is B, 14, which might be chosen if one forgets to take the reciprocal, but that gives the slope of the tangent to f, not to f⁻¹. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

4

Let $f(x)=x^3+2x-5$ and $f(2)=7$. If $g=f^{-1}$, what is $g'(7)$?

$\dfrac{1}{14}$

$\dfrac{1}{7^2+2}$

$\dfrac{1}{f'(7)}$

$14$

$f'(2)$

Explanation

This problem requires finding the derivative of an inverse function using the inverse function derivative theorem. Since g = f^(-1) and f(2) = 7, we know that g(7) = 2. The key relationship is that g'(7) = 1/f'(g(7)) = 1/f'(2). To find f'(2), we differentiate f(x) = x³ + 2x - 5 to get f'(x) = 3x² + 2, so f'(2) = 3(4) + 2 = 14. Therefore, g'(7) = 1/14. A common error is computing f'(7) instead of f'(2), which would give 1/149 rather than the correct 1/14. Remember: for inverse function derivatives, always evaluate f' at the point that maps to your input, not at the input itself.

5

Suppose $f(x)=\sin x+x$ and $f(0)=0$. If $g=f^{-1}$, what is $g'(0)$?

$\cos(0)+1$

$\dfrac{1}{\cos(g(0))+1}$

$\dfrac{1}{2}$

$2$

$\dfrac{1}{\cos(0)+1}$

Explanation

This problem requires finding the derivative of an inverse function at a specific point. Given f(0) = 0 and g = f^(-1), we have g(0) = 0. The inverse derivative formula gives us g'(0) = 1/f'(g(0)) = 1/f'(0). To find f'(x) from f(x) = sin(x) + x, we differentiate to get f'(x) = cos(x) + 1, so f'(0) = cos(0) + 1 = 1 + 1 = 2. Thus g'(0) = 1/2. Students might mistakenly think the answer is 2 by computing cos(0) + 1 directly without taking the reciprocal. The strategy: always remember that inverse function derivatives involve taking the reciprocal of the original function's derivative.

6

Let $f(x)=\ln(x)+x$ with $f(1)=1$. If $g=f^{-1}$, what is $g'(1)$?

$\dfrac{1}{2}$

$\dfrac{1}{1+\frac{1}{1}}$

$1+\dfrac{1}{1}$

$\dfrac{1}{1+\frac{1}{g(1)}}$

$2$

Explanation

To find the derivative of the inverse function g at x = 1, we use the inverse derivative relationship. Given f(1) = 1 and g = f^(-1), we know g(1) = 1. The formula g'(1) = 1/f'(g(1)) = 1/f'(1) applies here. Differentiating f(x) = ln(x) + x yields f'(x) = 1/x + 1, so f'(1) = 1/1 + 1 = 2. Therefore, g'(1) = 1/2. A tempting error is to substitute into the expression 1 + 1/g(1), which would give 2 rather than 1/2. Always remember: the derivative of an inverse function equals the reciprocal of the original function's derivative, evaluated at the appropriate point.

7

Let $f$ be differentiable and one-to-one with $f(2)=5$ and $f'(2)=3$; what is $(f^{-1})'(5)$?

$\dfrac{1}{5}$

$3$

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

$\dfrac{1}{3}$

$\dfrac{1}{f'(5)}$

Explanation

This problem requires finding the derivative of an inverse function at a specific point. The key relationship for inverse function derivatives is $ (f^{-1})'(b) = \frac{1}{f'(a)} $ where $ f(a) = b $. Since we're given $ f(2) = 5 $ and $ f'(2) = 3 $, we need $ (f^{-1})'(5) $. Because $ f(2) = 5 $, we know that $ f^{-1}(5) = 2 $, so we evaluate at $ a = 2 $. Therefore, $ (f^{-1})'(5) = \frac{1}{f'(2)} = \frac{1}{3} $. Choice A ($ 3 $) is tempting because it's $ f'(2) $, but the inverse derivative requires the reciprocal. Remember: to find $ (f^{-1})' $ at a point, take the reciprocal of $ f' $ at the corresponding input value.

8

A differentiable one-to-one function $p$ satisfies $p(0)=7$ and $p'(0)=5$; compute $(p^{-1})'(7)$.

$\dfrac{1}{p'(7)}$

$\dfrac{1}{5}$

$5$

$\dfrac{1}{7}$

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

Explanation

This problem involves finding the derivative of an inverse function at a specific value. The inverse function derivative formula tells us $(p^{-1})'(b) = rac{1}{p'(a)}$ when $p(a) = b$. We have $p(0) = 7$ and $p'(0) = 5$, and need $(p^{-1})'(7)$. Since $p(0) = 7$, we get $p^{-1}(7) = 0$, so we use $a = 0$. Thus $(p^{-1})'(7) = rac{1}{p'(0)} = rac{1}{5}$. Choice B ($5$) represents $p'(0)$ without the reciprocal, a common error. To find an inverse derivative: locate the input that produces your output, then take the reciprocal of the derivative there.

9

If $u$ is differentiable and one-to-one with $u(8)=2$ and $u'(8)=0.1$, what is $(u^{-1})'(2)$?

$\dfrac{1}{0.1}$

$\dfrac{1}{u'(8)}$

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

$\dfrac{1}{8}$

$0.1$

Explanation

To find the derivative of an inverse function, apply $(u^{-1})'(b) = rac{1}{u'(a)}$ when $u(a) = b$. Given $u(8) = 2$ and $u'(8) = 0.1$, we seek $(u^{-1})'(2)$. Since $u(8) = 2$, we have $u^{-1}(2) = 8$, so we evaluate at $a = 8$. Therefore, $(u^{-1})'(2) = rac{1}{u'(8)} = rac{1}{0.1} = 10$. Choice A ($0.1$) represents $u'(8)$ itself rather than its reciprocal. The consistent pattern: inverse derivatives equal one divided by the original derivative at the pre-image point.

10

A one-to-one differentiable function $g$ satisfies $g(-1)=4$ and $g'(-1)=-2$; find $(g^{-1})'(4)$.

$\dfrac{1}{2}$

$-2$

$-\dfrac{1}{2}$

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

$\dfrac{1}{4}$

Explanation

This question asks for the derivative of an inverse function using the inverse function derivative formula. The fundamental relationship states that $ (g^{-1})'(b) = \frac{1}{g'(a)} $ when $ g(a) = b $. We're told $ g(-1) = 4 $ and $ g'(-1) = -2 $, and we need $ (g^{-1})'(4) $. Since $ g(-1) = 4 $, we have $ g^{-1}(4) = -1 $, so we use $ a = -1 $ in our formula. Thus $ (g^{-1})'(4) = \frac{1}{g'(-1)} = \frac{1}{-2} = -\frac{1}{2} $. Choice A ($ -2 $) incorrectly uses $ g'(-1) $ directly without taking the reciprocal. The strategy: identify which original input gives your desired output, then take the reciprocal of the derivative at that input.

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