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Challenge

Undoing a Function

Consider a function that takes any real number and returns a number greater by two units.
input and output
What would be a function that undoes

Discussion

Inverse Relation

An inverse relation is a relation that interchanges the input and output values of a specific relation. In other words, it is the relation that undoes a certain relation. Consider the mapping diagram that shows a relation that takes elements from set and returns elements from set
relation and inverse relation
The relation that undoes is In this case, takes elements from set and returns elements from set Relations can also be represented by a collection of ordered pairs. Here, the inverse of a relation is obtained by interchanging the elements of the ordered pairs of the original relation.
Graphically, the inverse of a relation is a reflection across the line
inverse function
If the inverse relation of a function is also a function, then it is called an inverse function.

Discussion

Inverse of a Function

In general, the inverse of a linear function is also a linear function.

Example

Using Graphs to Determine if Two Functions Are Inverse Functions

Kriz enjoys playing video games with their friends. For their birthday they received a copy of Mathleaks: The Adventure, a math-based video game.

video games
Kriz is very close to beating this level. They need to graph the functions given below and determine whether they are inverse functions.
Help Kriz beat the level and get one step closer beating the game by mastering math!

Answer

Graph:

inverse functions

Are and Inverse Functions? Yes.

Hint

Graph both functions on the same coordinate plane and see if they are each other's reflection across the line

Solution

Since and are linear functions written in slope-intercept form, they can both be graphed using their slope and intercept.

graph of both f and g
Now, it can be seen whether the lines are each other's reflection across
graph of both f and g
The lines are indeed each other's reflection across Therefore, and are inverse functions.

Pop Quiz

Determining if Two Functions Are Inverses by Looking at Their Graphs

It was discussed previously that if the graphs of two functions are each other's reflection across then they are inverse functions. Therefore, it can be determined if two functions are inverse just by looking at their graphs. Determine whether the following linear functions are inverse by looking at the lines.

Do the lines shown correspond to inverse functions?

Example

Verifying Inverse Functions by Composition

If Paulina gets an A on a math test, her mother will buy her a new saxophone.

saxophone

To get an A, Paulina must answer two questions correctly. Help her get the new saxophone!

a Given and calculate and and determine whether and are inverse functions.
Are and inverse functions?
b Given and calculate and and determine whether and are inverse functions.
Are and inverse functions?

Hint
a If and then and are inverse functions.
b If and then and are inverse functions.

Solution
a Consider the given functions.
The composite function will be found first. To do so, the definition of will be used.
Next, can be substituted in the above equation.
Substitute

Simplify right-hand side
Distr

Distribute

AssociativePropMult

\AssociativePropMult

DenomMultFracToNumber

Multiply

Multiply

AddTerms

Add terms

By following the same procedure, an expression for can be found.
Definition of First Function Substitute Second Function Simplify

It has been found that both and are equal to Therefore, and are inverse functions.

b Consider the given functions.
By following the same procedure as in Part A, the expressions for and will be found.
Definition of First Function Substitute Second Function Simplify

It has been found that neither nor is equal to Therefore, and are not inverse functions.

Pop Quiz

Determining if Two Functions Are Inverse by Composition

For the functions and use a composition to determine whether they are inverse functions.

determine if two functions are inverse by compounding

Discussion

Finding the Inverse of a Function Algebraically

A function can be represented by a table of values, a graph, a mapping diagram, or a function rule, among other ways. Depending on how the function is presented, finding its inverse can be done in different ways. When a function rule is given, finding the inverse algebraically is advantageous. Consider the following example function.
There is a series of steps to follow in order to find the inverse function
1
Replace With
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To begin, since describes the input-output relationship of the function, replace with in the function rule.
2
Switch and
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Because the inverse of a function reverses and the variables can be switched. Notice that every other piece in the function rule remains the same.
3
Solve for
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Solve the resulting equation from the previous step for This will involve using the inverse operations.
Solve for
MultEqn

AddEqn

DivEqn

RearrangeEqn

Rearrange equation

4
Replace With
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Just as shows the input-output relationship of so does Therefore, replacing with gives the rule for the inverse of
Notice that in the input is multiplied by decreased by and divided by From the rule of it can be seen that undergoes the inverse of these operations in the reverse order. Specifically, is multiplied by increased by and divided by

Example

Finding the Inverse of a Function

Zosia lives in Honolulu, Hawaii.

beach
She is obsessed with math. While watching a surfer catch some gnarly waves, she came up with a function that models the distance of the surfer from the shore.
Here, is the distance of the surfer from the shore after seconds of riding a wave. Given the distance from shore, Zosia now wants to finds the function that gives the time, in seconds, the surfer has been riding a wave. To do so, she needs to find the inverse of Help Zosia find this function! Write the answer in slope-intercept form.

Hint

Start by replacing with After that, switch the and variables. Then, solve the obtained equation for Finally, replace with

Solution

To find the inverse of the function, there are four steps to follow.

  1. Replace with
  2. Switch and
  3. Solve the obtained equation for
  4. Replace with

These steps will be done one at a time.

Step

Here, the variable will be substituted for in the given function.

Step

In this step, the variables and must be switched.

Step

Here, the equation obtained in the previous step will be solved for
Solve for
SubEqn

MultEqn

Distr

Distribute

CommutativePropMult

\CommutativePropMult

MultPosNeg

MoveLeftFacToNum

CalcQuot

Calculate quotient

SubNeg

RearrangeEqn

Rearrange equation

Step

Finally, will be substituted for the variable in the equation obtained in the previous step.

Pop Quiz

Finding the Inverse Function

Find the inverse of the given function. Write the answer in slope-intercept form — in the format If the slope and intercept are not integers, express them as decimal numbers. If necessary, round them to two decimal places.

finding the inverse of a linear function

Closure

Undoing a Function

In this lesson, it has been seen that the function that undoes a function is its inverse Consider the function given in the challenge presented at the beginning of the lesson.
input and output
Find the inverse function of

Hint

Start by replacing with After that, switch the and variables. Then, solve the obtained equation for and finally replace with

Solution

To find the inverse of the function, there are four steps to follow.

  1. Replace with
  2. Switch and
  3. Solve the obtained equation for
  4. Replace with

These steps will be done one at a time.

Step

Here, the variable will be substituted for in the given function.

Step

In this step, the variables and must be switched.

Step

Here, the equation obtained in the previous step will be solved for

Step

Finally, will be substituted for the variable in the equation obtained in the previous step.
It has been found that the inverse function of and therefore the function that undoes it, is
input and output
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