INVERSE

FUNCTIONSBy

JUNREY P. FALCON

Set X Set Y

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Remember we talked about functions---taking a set X and mapping into a Set Y

An inverse function would reverse that process and map from SetY back into Set X

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123

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If we map what we get out of the function back, we won’t always have a function going back.

Since going back, 6 goes back to both 3 and 5, the mapping going back is NOT a function

These functions are called many-to-one functions

Only functions that pair the y value (value in the range) with only one x will be functions going back

the other way. These functions are called one-to-one functions.

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This would not be a one-to-one function because to be one-to-one, each y would only be used once with an x.

This is a function IS one-to-one. Each x is paired with only one y and each y is paired with only one x

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Only one-to-one functions will have inverse functions, meaning the mapping back to the original values is also

a function.

Recall that to determine by the graph if an equation is a function, we have the vertical line test.

If a vertical line intersects the graph of an equation more than one time, the equation graphed is NOT a function.

This is a functionThis is NOT a

function This is a function

To be a one-to-one function, each y value could only be paired with one x. Let’s look at a couple of graphs.

Look at a y value (for example y = 3)and see if there is only one x value on the graph for it.

This is a many-to-one function

For any y value, a horizontal line will only intersection the graph once so will only have one x value

This then IS a one-to-one function

If a horizontal line intersects the graph of an equation more than one time, the equation graphed is NOT a one-to-one function and will NOT have an inverse function.

This is a one-to-one

function

This is NOT a one-to-one

function

This is NOT a one-to-one function

Let’s consider the function and compute some values and graph them.

3xxf

x f (x)

-2 -8-1 -1 0 0 1 1 2 8

Is this a one-to-one function? Yes, so it will have an inverse function

What will “undo” a cube? A cube root

31 xxf

This means “inverse function”

x f -1(x)

-8 -2-1 -1 0 0 1 1 8 2

Let’s take the values we got out

of the function and put them into the inverse function

and plot them

Notice that the x and y values traded places for the function

and its inverse.

These functions are reflections of each other

about the line y = x

3xxf

31 xxf

(2,8)

(8,2)

(-8,-2)

(-2,-8)

So geometrically if a function and its inverse are graphed, they are reflections about the line y = x and the

x and y values have traded places. The domain of the function is the range of the inverse. The range of the function is the domain of the inverse. Also if we start

with an x and put it in the function and put the result in the inverse function, we are back where we started from.

Given two functions, we can then tell if they are inverses of each other if we plug one into the other and it

“undoes” the function. Remember subbing one function in the other was the composition function.

xxgfgf xxfgfg

So if f and g are inverse functions, their composition would simply give x back. For inverse functions then:

Verify that the functions f and g are inverses of each other.

2;2,2 2 xxgxxxf

If we graph (x - 2)2 it is a parabola shifted right 2.

Is this a one-to-one function?

This would not be one-to-one but they restricted the domain

and are only taking the function where x is greater

than or equal to 2 so we will have a one-to-one function.

Verify that the functions f and g are inverses of each other.

2;2,2 2 xxgxxxf

xxxgf 22

22

xxxfg 2222 2

Since both of these = x, if you start with x and apply the functions they “undo” each other and are inverses.

Steps for Finding the Inverse of a One-to-One Function

Replace f(x) with y

Trade x and y places

Solve for y

y = f -1(x)

Find the inverse of

Replace f(x) with y

Trade x and y places

Solve for y

y = f -1(x)

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xf

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42 yx42 xyx

xxy 24

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xy

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xxf

241

Let’s check this by doing 1ff

xx

ff42

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xxx 422

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xYes!

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xxf

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or

Ensure f(x) is one to one first. Domain may need to be restricted.

Acknowledgement

I wish to thank Shawna Haider from Salt Lake Community College, Utah USA for her hard work in creating this PowerPoint.

www.slcc.edu

Shawna has kindly given permission for this resource to be downloaded from www.mathxtc.com and for it to be modified to suit the Western Australian Mathematics Curriculum.

Stephen CorcoranHead of MathematicsSt Stephen’s School – Carramarwww.ststephens.wa.edu.au