Differentiating Inverse Functions

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AP Calculus AB › Differentiating Inverse Functions

Questions 1 - 10
1

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

$\dfrac{1}{9}$

$f'(2)$

$12$

$\dfrac{1}{12}$

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

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.

2

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

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

$\dfrac{1}{3}$

$\dfrac{1}{4}$

$g'(1)$

$4$

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.

3

If $t(x)=x^3+3x$ and $t(1)=4$, what is $(t^{-1})'(4)$?

$\dfrac{1}{4}$

$t'(1)$

$\dfrac{1}{t'(4)}$

$6$

$\dfrac{1}{6}$

Explanation

This problem tests the skill of differentiating inverse functions. The derivative of the inverse function at a point $a$ is given by $ \frac{1}{\text{derivative of the original function at } b} $, where $ t(b) = a $. Here, $ t(1) = 4 $, so $ b = 1 $ and $ a = 4 $. Compute $ t'(x) = 3x^2 + 3 $, so $ t'(1) = 6 $, thus $ (t^{-1})'(4) = \frac{1}{6} $. A tempting distractor is A, $ 6 $, which might be chosen if one forgets to take the reciprocal, but that gives the slope of the tangent to $ t $, not to $ t^{-1} $. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

4

Let $f(x)=x+\dfrac{1}{x}$ for $x>0$ and $f(1)=2$. Find $(f^{-1})'(2)$.

$f'(1)$

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

$\dfrac{1}{2}$

$\dfrac{1}{0}$

$0$

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(1) = 2, so b = 1 and a = 2. Compute f'(x) = 1 - 1/x², so f'(1) = 0, thus (f⁻¹)'(2) = 1/0, which is undefined. A tempting distractor is A, 0, which might be chosen if one mistakenly takes the reciprocal of 0 as 0, but actually it indicates the inverse is not differentiable there. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

5

Given $r(x)=\ln(x+1)$ and $r(e-1)=1$, find $(r^{-1})'(1)$.

$r'(e-1)$

$\dfrac{1}{r'(e)}$

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

$\dfrac{1}{e}$

$e$

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 $r(b) = a$. Here, $r(e-1) = 1$, so b = e-1 and a = 1. Compute $r'(x) = 1/(x+1)$, so $r'(e-1) = 1/e$, thus $(r^{-1})'(1) = e$. A tempting distractor is A, $1/e$, which might be chosen if one forgets to take the reciprocal, but that gives the slope of the tangent to r, not to r⁻¹. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

6

Let $f(x)=x^3-4x$ and $f(2)=0$. If $g=f^{-1}$, what is $g'(0)$?

$f'(2)$

$\dfrac{1}{3\cdot2^2-4}$

$\dfrac{1}{8}$

$8$

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

Explanation

To find the derivative of the inverse function g at x = 0, we use the inverse derivative relationship. Since f(2) = 0 and g = f^(-1), we know g(0) = 2. The formula g'(0) = 1/f'(g(0)) = 1/f'(2) applies. Differentiating f(x) = x³ - 4x gives f'(x) = 3x² - 4, so f'(2) = 3(4) - 4 = 12 - 4 = 8. Therefore, g'(0) = 1/8. A common error is computing f'(0) = -4 and getting -1/4, but we must evaluate f' at the point g(0) = 2. Remember: the input to g' determines where we evaluate f', not the input value itself.

7

Suppose $v$ is differentiable and one-to-one with $v(8)=-3$ and $v'(8)=\tfrac{5}{9}$. Find $(v^{-1})'(-3)$.

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

$\dfrac{1}{v'(-3)}$

$\dfrac{9}{5}$

$-\dfrac{9}{5}$

$\dfrac{5}{9}$

Explanation

This question tests understanding of inverse function differentiation. The key formula is $(v^{-1})'(b) = \frac{1}{v'(a)}$ where $v(a) = b$. Since $v(8) = -3$, we know $v^{-1}(-3) = 8$, and we need $(v^{-1})'(-3)$. Applying the formula with $a = 8$ and $b = -3$: $(v^{-1})'(-3) = \frac{1}{v'(8)} = \frac{1}{5/9} = \frac{9}{5}$. Choice B ($\frac{5}{9}$) is $v'(8)$ without reciprocation, a frequent error. Remember: inverse derivatives require reciprocating the original function's derivative at the pre-image point.

8

If $g(x)=e^x+x$ and $g(0)=1$, what is $(g^{-1})'(1)$?

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

$\dfrac{1}{1}$

$g'(0)$

$\dfrac{1}{2}$

$2$

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 g(b) = a. Here, g(0) = 1, so b = 0 and a = 1. Compute g'(x) = eˣ + 1, so g'(0) = 2, thus (g⁻¹)'(1) = 1/2. A tempting distractor is A, 2, 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.

9

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

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

$\dfrac{1}{7}$

$f'(2)$

$14$

$\dfrac{1}{14}$

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.

10

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

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

$\dfrac{1}{2}$

$q'(1)$

$\dfrac{1}{5}$

$5$

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 q(b) = a. Here, q(1) = 2, so b = 1 and a = 2. Compute q'(x) = 3x² + 2x, so q'(1) = 5, thus (q⁻¹)'(2) = 1/5. A tempting distractor is A, 5, which might be chosen if one forgets to take the reciprocal, but that gives the slope of the tangent to q, not to q⁻¹. To solve similar problems, always remember that the derivatives of a function and its inverse are reciprocals at corresponding points.

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