Precalculus : Inverse Functions

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #1 : Find The Inverse Of A Relation

What is the inverse function of

\displaystyle y=3x+5 ?

Possible Answers:

\displaystyle y=x-5

\displaystyle y=\frac{x}{3}

\displaystyle y=3x+5

\displaystyle y=\frac{x-3}{5}

\displaystyle y=\frac{x-5}{3}

Correct answer:

\displaystyle y=\frac{x-5}{3}

Explanation:

To find the inverse function of

\displaystyle y=3x+5

we replace the \displaystyle y with \displaystyle x and vice versa.

So

\displaystyle x=3y+5

Now solve for \displaystyle y

\displaystyle x-5=3y

\displaystyle 3y=x-5

\displaystyle \frac{3y}{3}=\frac{x-5}{3}

\displaystyle y=\frac{x-5}{3}

Example Question #2 : Find The Inverse Of A Relation

Find the inverse of the function.

 \displaystyle f(x)= \sin{\frac{x^2}{10}}

Possible Answers:

\displaystyle f^{-1}(x)=\sqrt{ 10 \arcsin{x} }

\displaystyle f^{-1}(x)=10 \sqrt{ \arcsin{y} }

\displaystyle f^{-1}(x)=\arcsin{ \frac{x^2}{10} }

\displaystyle f^{-1}(x)=\sqrt{ 10 \cos{x} }

Correct answer:

\displaystyle f^{-1}(x)=\sqrt{ 10 \arcsin{x} }

Explanation:

To find the inverse function, first replace \displaystyle f(x) with \displaystyle y:

\displaystyle y= \sin{ \frac{x^2}{10} }

Now replace each \displaystyle y with an \displaystyle x and each \displaystyle x with a \displaystyle y:

\displaystyle x= \sin{ \frac{y^2}{10} }

Solve the above equation for \displaystyle y:

\displaystyle \arcsin{x}= \frac{y^2}{10}

\displaystyle \sqrt{10 \arcsin{x}} = y

Replace \displaystyle y with \displaystyle f^{-1}(x). This is the inverse function:

 

\displaystyle f^{-1}(x)=\sqrt{ 10 \arcsin{x} }

 

Example Question #2 : Linear Algebra

Find the inverse of the function.

 \displaystyle g(x)= \frac{x+3}{2x-4}

Possible Answers:

\displaystyle g^{-1}(x)= \frac{3x+4}{x-2}

\displaystyle g^{-1}(x)= \frac{x+3}{-4x+1}

\displaystyle g^{-1}(x)= \frac{4x+3}{2x-1}

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

Correct answer:

\displaystyle g^{-1}(x)= \frac{4x+3}{2x-1}

Explanation:

To find the inverse function, first replace \displaystyle g(x) with \displaystyle y:

\displaystyle y= \frac{x+3}{2x-4}

Now replace each \displaystyle y with an \displaystyle x and each \displaystyle x with a \displaystyle y:

\displaystyle x= \frac{y+3}{2y-4}

Solve the above equation for \displaystyle y:

\displaystyle x(2y-4)=y+3

\displaystyle 2xy-4x=y+3

\displaystyle 2xy-y=4x+3

\displaystyle (2x-1)y=4x+3

\displaystyle y= \frac{4x+3}{2x-1}

Replace \displaystyle y with \displaystyle g^{-1}(x). This is the inverse function:

\displaystyle g^{-1}(x)= \frac{4x+3}{2x-1}

Example Question #3 : Find The Inverse Of A Relation

Find the inverse of the function \displaystyle y=2x^2+1.

Possible Answers:

Correct answer:

Explanation:

To find the inverse of \displaystyle y=2x^2+1, interchange the \displaystyle x and \displaystyle y terms and solve for \displaystyle y.

\displaystyle y=2x^2+1

\displaystyle x=2y^2+1

\displaystyle x-1=2y^2

\displaystyle \frac{x-1}{2}=y^2

Example Question #1 : Find The Inverse Of A Relation

What point is the inverse of the \displaystyle (2,3)?

Possible Answers:

\displaystyle (2,-3)

\displaystyle (3,2)

\displaystyle (-2,-3)

\displaystyle (-2,3)

Correct answer:

\displaystyle (3,2)

Explanation:

When trying to find the inverse of a point, switch the x and y values.

So \displaystyle (2,3) \displaystyle \rightarrow (3,2)

Example Question #2 : Find The Inverse Of A Relation

What is the inverse of \displaystyle \left(\frac{1}{2},3\right)?

Possible Answers:

\displaystyle \left(-3,-\frac{1}{2}\right)

\displaystyle \left(-3,\frac{1}{2}\right)

\displaystyle \left(3,-\frac{1}{2}\right)

\displaystyle \left(3,\frac{1}{2}\right)

Correct answer:

\displaystyle \left(3,\frac{1}{2}\right)

Explanation:

When trying to find the inverse of a point, switch the x and y values.

So, \displaystyle \left(\frac{1}{2},3\right)\rightarrow \left(3, \frac{1}{2} \right )

 

Example Question #1 : Inverse Functions

Find the inverse of the following function:

\displaystyle y=4x+10 

Possible Answers:

\displaystyle y=\frac{x-4}{10}

\displaystyle y=\frac{x-10}{4}

\displaystyle y=4x-10

\displaystyle y=10x-4

\displaystyle y=\frac{x}{4}

Correct answer:

\displaystyle y=\frac{x-10}{4}

Explanation:

In order to find the inverse of the function, we need to switch the x- and y-variables.

\displaystyle y=4x+10

After switching the variables, we have the following:

\displaystyle x=4y+10

Now solve for the y-variable. Start by subtracting 10 from both sides of the equation.

\displaystyle x-10=4y+10-10

\displaystyle x-10=4y

Divide both sides of the equation by 4.

\displaystyle \frac{x-10}{4}=\frac{4y}{4}

Rearrange and solve.

\displaystyle y=\frac{x-10}{4}

Example Question #1 : Inverse Functions

Find the inverse of,

 \displaystyle 3x+1=y.

Possible Answers:

\displaystyle x=3y+1

\displaystyle x=\frac{y-1}{3}

\displaystyle y=\frac{x+1}{3}

\displaystyle y=\frac{x-1}{3}

Correct answer:

\displaystyle y=\frac{x-1}{3}

Explanation:

In order to find the inverse, switch the x and y variables in the function then solve for y.

\displaystyle 3x+1=y

Switching variables we get,

 \displaystyle 3y+1=x.

 Then solving for y to get our final answer.

\displaystyle 3y=x-1

\displaystyle y=\frac{x-1}{3}

Example Question #2 : Inverse Functions

Find the inverse of,

\displaystyle y=x^{2}.

Possible Answers:

\displaystyle y=\frac{x}{2}

\displaystyle x=\pm \sqrt{y}

\displaystyle y=\pm\sqrt{x}

\displaystyle x=y^{2}

Correct answer:

\displaystyle y=\pm\sqrt{x}

Explanation:

First, switch the variables making \displaystyle y=x^2 into \displaystyle x=y^{2}.

Then solve for y by taking the square root of both sides.

\displaystyle y^2=x

\displaystyle \sqrt {y^2}=\pm \sqrt x

\displaystyle y=\pm \sqrt x

 

Example Question #1 : Find The Inverse Of A Function

Find the inverse of the following equation.

\displaystyle y=\sqrt{\ln(x)+2}.

Possible Answers:

\displaystyle y=\pm x^2 -2

\displaystyle y=e^x+2

\displaystyle y=x-2

Correct answer:

Explanation:

To find the inverse in this case, we need to switch our x and y variables and then solve for y.

Therefore,

\displaystyle y=\sqrt{\ln(x)+2} becomes,

\displaystyle x=\sqrt{\ln(y)+2}

To solve for y we square both sides to get rid of the sqaure root.

\displaystyle x^2=\ln(y)+2

We then subtract 2 from both sides and take the exponenetial of each side, leaving us with the final answer.

\displaystyle x^2-2=\ln(y)

\displaystyle e^{x^2-2}=e^{\ln(y)}

 

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