Calculus AB : Differentiate Inverse Functions

Study concepts, example questions & explanations for Calculus AB

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Example Questions

Example Question #1 : Differentiating Functions

Which expression correctly identifies the inverse of \(\displaystyle f (x) = 2x-4\)?

Possible Answers:

\(\displaystyle f^{-1}(x) =\frac{1}{2}x+2\)

 

\(\displaystyle f^{-1}(x) =x+2\)

\(\displaystyle f^{-1}(x) =\frac{1}{2}x+4\)

\(\displaystyle f^{-1}(x) =\frac{1}{2}x-2\)

Correct answer:

\(\displaystyle f^{-1}(x) =\frac{1}{2}x+2\)

 

Explanation:

The inverse of a function can be found by substituting yvariables for the \(\displaystyle x\) variables found in the function, then setting the function equal to \(\displaystyle x\). By next isolating \(\displaystyle y\), the inverse function is written. Then, the notation \(\displaystyle f^{-1}\)is used to describe the newly written function as being the inverse of the original function. The answer choice “\(\displaystyle f^{-1}(x) =\frac{1}{2}x+2\)” is correct.

Example Question #2 : Differentiating Functions

Which of the following correctly identifies the derivative of an inverse function?

Possible Answers:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(y))}\)

\(\displaystyle (f^{-1})'(y) = \frac{1}{f'(f^{-1}(x))}\)

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\)

\(\displaystyle (f^{-1})'(x) = \frac{2}{f'(f^{-1}(x))}\)

Correct answer:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\)

Explanation:

This question asks you to recognize the correct notation of a differentiating inverse functions problem. First, it is key to recognize that the equation needs to have the same variable throughout, thus eliminating the answer choices \(\displaystyle (f^{-1})'(y) = \frac{1}{f'(f^{-1}(x)) }\)and \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(y))}\). Next, there should be no constants in the correct equation; thus, \(\displaystyle (f^{-1})'(x) = \frac{2}{f'(f^{-1}(x))}\) is incorrect. The correct choice is \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

Example Question #3 : Differentiating Functions

Find \(\displaystyle (f^{-1})'(1)\) given \(\displaystyle f (x) = x^5+2x+1\).

Possible Answers:

\(\displaystyle \frac{1}{3}\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle \frac{2}{3}\)

\(\displaystyle \frac{1}{4}\)

Correct answer:

\(\displaystyle \frac{1}{2}\)

Explanation:

Let \(\displaystyle f (x) = x^5+2x+1\)

It is important to recognize the relationship between a function and its inverse to solve.

If \(\displaystyle f (0) = (0)^5+2(0)+1=1\), solving for the inverse function will produce \(\displaystyle f^{-1}(1)=0\).

To find the derivative of an inverse function, use: \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\)

 

\(\displaystyle (f^{-1})'(1) = \frac{1}{f'(f^{-1}(1))}=\frac{1}{f' (0)}=\frac{1}{5(0)^4+ 2}=\frac{1}{0 + 2}=\frac{1}{2}\)

 

Therefore, \(\displaystyle (f^{-1})'(1)=\frac{1}{2}\)

Example Question #2 : Differentiating Functions

Let \(\displaystyle f (x) =x^6\).Find \(\displaystyle (f^{-1})'(x)\).

Possible Answers:

\(\displaystyle \frac{6}{x}\)

\(\displaystyle \frac{1}{6x^5_6}\)

\(\displaystyle \frac{1}{4x^5_6}\)

\(\displaystyle \frac{1}{6x}\)

Correct answer:

\(\displaystyle \frac{1}{6x^5_6}\)

Explanation:

To find the derivative of the inverse of \(\displaystyle f (x)\), it is useful to first solve for \(\displaystyle f^{-1}\).

This will help because \(\displaystyle f^{-1}\)is needed in the derivative equation, \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

\(\displaystyle f (x) =x^6 \rightarrow f^{-1} (x) =\sqrt[6]{x}\)

Next, the equation for the derivative of an inverse function can be evaluated.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(\sqrt[6]{x})}\)

After substituting in \(\displaystyle \sqrt[6]{x}\) for \(\displaystyle f^{-1} (x)\), the derivative of \(\displaystyle f (x),\) or \(\displaystyle f'(x) =6x^5\), is applied:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(\sqrt[6]{x})}=\frac{1}{6(\sqrt[6]{x})^5}=\frac{1}{6x^5_6}\)

Therefore, the correct answer is \(\displaystyle (f^{-1})'(x)=\frac{1}{6x^5_6}\)

Example Question #3 : Differentiating Functions

Let \(\displaystyle f (x) =e^x\). Find \(\displaystyle (f^{-1})'(x)\).

Possible Answers:

\(\displaystyle ln(x)\)

\(\displaystyle \frac{1}{x}\)

\(\displaystyle xe^{x-1}\)

\(\displaystyle e^x\)

Correct answer:

\(\displaystyle \frac{1}{x}\)

Explanation:

To find the derivative of the inverse of \(\displaystyle f (x)\), it is useful to first solve for \(\displaystyle f^{-1}\).

This will help because \(\displaystyle f^{-1}\) is needed in the derivative equation, \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

\(\displaystyle f (x) =e^x \rightarrow f^{-1} (x) =ln(x)\)

Next, the equation for the derivative of an inverse function can be evaluated.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(ln(x))}\)

After substituting in \(\displaystyle ln(x) for f^{-1} (x)\), the derivative of \(\displaystyle f (x)\), or\(\displaystyle f'(x) =e^x\), is applied:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(ln(x))}=\frac{1}{e^{ln(x)}}=\frac{1}{x}\)

Therefore, the correct answer is \(\displaystyle (f^{-1})'(x)=\frac{1}{x}\)

Example Question #6 : Differentiating Functions

Let \(\displaystyle f (x) =\sqrt[3]{x}\). Find \(\displaystyle (f^{-1})'(x)\).

Possible Answers:

\(\displaystyle 2x^3\)

\(\displaystyle \frac{3}{x^2}\)

\(\displaystyle 3x^2\)

\(\displaystyle \frac{1}{x^2}\)

Correct answer:

\(\displaystyle 3x^2\)

Explanation:

To find the derivative of the inverse of \(\displaystyle f (x)\), it is useful to first solve for \(\displaystyle f^{-1}\).

This will help because \(\displaystyle f^{-1}\)is needed in the derivative equation, \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

\(\displaystyle f (x)=\sqrt[3]{x} \rightarrow f^{-1} (x)=x^3\)

Next, the equation for the derivative of an inverse function can be evaluated.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(x^3)}\)

After substituting in \(\displaystyle x^3\) for \(\displaystyle f^{-1} (x)\), the derivative of \(\displaystyle f (x)\), or \(\displaystyle f'(x)=(\frac{1}{3})(x^{-\frac{2}{3}})\), is applied:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{(\frac{1}{3})(x^3)^{-\frac{2}{3}}}=\frac{3}{x^{-2}}=3x^2\)

Therefore, the correct answer is \(\displaystyle (f^{-1})'(x)=3x^2\).

Example Question #4 : Differentiating Functions

Let \(\displaystyle f (x) =2-2x\). Find \(\displaystyle (f^{-1})'(x)\).

Possible Answers:

\(\displaystyle -\frac{1}{2}\)

\(\displaystyle \frac{1}{3}\)

\(\displaystyle -3\)

\(\displaystyle 2x\)

Correct answer:

\(\displaystyle -\frac{1}{2}\)

Explanation:

To find the derivative of the inverse of \(\displaystyle f (x)\), it is useful to first solve for \(\displaystyle f^{-1}\).

This will help because \(\displaystyle f^{-1}\)is needed in the derivative equation, \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

\(\displaystyle f (x)=2-2x \rightarrow f^{-1} (x)=1-\frac{1}{2}x\)

Next, the equation for the derivative of an inverse function can be evaluated.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(1-\frac{1}{2}x)}\)

After substituting in \(\displaystyle 1-\frac{1}{2}x\) for \(\displaystyle f^{-1} (x)\), the derivative of \(\displaystyle f (x)\), or \(\displaystyle f'(x)= -2\), is applied:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}= -\frac{1}{2}\)

Therefore, the correct answer is \(\displaystyle (f^{-1})'(x)=-\frac{1}{2}\).

Example Question #5 : Differentiating Functions

Let \(\displaystyle f (x) =6x^2\). Find \(\displaystyle (f^{-1})'(x)\).

Possible Answers:

\(\displaystyle 6x\)

\(\displaystyle \frac{1}{12x^2}\)

\(\displaystyle \frac{1}{12(\sqrt{}^x_6)}\)

\(\displaystyle \frac{1}{2x}\)

Correct answer:

\(\displaystyle \frac{1}{12(\sqrt{}^x_6)}\)

Explanation:

To find the derivative of the inverse of \(\displaystyle f (x)\), it is useful to first solve for \(\displaystyle f^{-1}\).

This will help because \(\displaystyle f^{-1}\)is needed in the derivative equation, \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

\(\displaystyle f (x) =6x^2 \rightarrow f^{-1} (x) =\sqrt[2]{^x_6}\)

Next, the equation for the derivative of an inverse function can be evaluated.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(\sqrt[2]{^x_6})}\)

After substituting in \(\displaystyle \sqrt[2]{^x_6}\) for \(\displaystyle f^{-1} (x)\), the derivative of \(\displaystyle f (x)\), or \(\displaystyle f'(x) =12x\), is applied:

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(\sqrt[2]{^x_6})}=\frac{1}{12(\sqrt[2]{^x_6})}\)

Therefore, the correct answer is \(\displaystyle (f^{-1})'(x)=\frac{1}{12(\sqrt{^x_6})}\)

Example Question #1 : Differentiate Inverse Functions

Suppose the points in the table below represent the continuous function \(\displaystyle f(x)\). The differentiable function \(\displaystyle g(x)\) is the inverse of the function \(\displaystyle f(x)\). Find \(\displaystyle g'(2)\).

Q9 table

Possible Answers:

\(\displaystyle \frac{1}{6}\)

\(\displaystyle 6\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle \frac{1}{6}\)

Explanation:

Below is the equation for the derivative of \(\displaystyle g(x)\):

\(\displaystyle g'(x) = \frac{1}{f'(f^{-1}(x))}\)

\(\displaystyle g'(2) = \frac{1}{f'(f^{-1}(2))}\)

So, the value of \(\displaystyle f^{-1}(2)\)must first be found.

Using the data from the table, \(\displaystyle f^{-1}(2)=2\) since \(\displaystyle f(2)=2\).

Next, from the table the following can be obtained: 

\(\displaystyle f'(2)=6\)

Now, the appropriate substitutions can be made to solve for \(\displaystyle g'(2)\).

\(\displaystyle g'(2)=\frac{1}{f'(f^{-1}(2))}=\frac{1}{f'(2)}=\frac{1}{6}\)

Example Question #10 : Differentiating Functions

Find \(\displaystyle (f^{-1})'(3)\) given \(\displaystyle f (x) =ln(3x)\).

Possible Answers:

\(\displaystyle \frac{1}{3}e^9\)

\(\displaystyle \frac{1}{3}e^3\)

\(\displaystyle e^9\)

\(\displaystyle e^3\)

Correct answer:

\(\displaystyle \frac{1}{3}e^3\)

Explanation:

To find the derivative of the inverse of \(\displaystyle f (x)\), it is useful to first solve for \(\displaystyle f^{-1}\).

This will help because \(\displaystyle f^{-1}\) is needed in the derivative equation,  \(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}\).

\(\displaystyle f (x) =ln(3x) \rightarrow f^{-1} (x) =\frac{1}{3}e^x\)

Next, the equation for the derivative of an inverse function can be evaluated.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(\frac{1}{3}e^x)}\)

After substituting in \(\displaystyle \frac{1}{3}e^x\) for \(\displaystyle f^{-1} (x)\), the derivative of \(\displaystyle f (x)\), or \(\displaystyle f'(x) =\frac{1}{x}\), is found by taking the derivative of \(\displaystyle ln(3x)\) and applying chain rule.

\(\displaystyle (f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}=\frac{1}{f'(\frac{1}{3}e^x)}=\frac{1}{\frac{1}{\frac{1}{3}e^x}}=\frac{\frac{1}{3}e^x}{1}=\frac{1}{3}e^x\)

After finding the general term \(\displaystyle (f^{-1})'(x)=\frac{1}{3}e^x\), evaluate at \(\displaystyle x=3\).

\(\displaystyle (f^{-1})'(3)=\frac{1}{3}e^3\)

Therefore, the correct answer is \(\displaystyle (f^{-1})'(3)=\frac{1}{3}e^3\).

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