5.4 functions and their inverses cc.9-12.f.bf.1c (+) compose functions. cc.9-12.a.ced.2 create...

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context. Warm Up

Solve for x in terms of y.

1.

2.

3.

4. y = 2ln x

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context.

Determine whether the inverse of a function is a function.

Write rules for the inverses of functions.

Objectives


one-to-one function

Vocabulary


You learned that the inverse of a function f(x) “undoes” f(x). Its graph is a reflection across line y = x. The inverse may or not be a function.

Recall that the vertical-line test can help you determine whether a relation is a function. Similarly, the horizontal-line test can help you determine whether the inverse of a function is a function.


Use the horizontal-line test to determine whether the inverse of the blue relation is a function.

Example 1

The inverse is a function because no horizontal line passes through two points on the graph.


Use the horizontal-line test to determine whether the inverse of the red relation is a function.

Example 2

The inverse is a not a function because a horizontal line passes through more than one point on the graph.


Use the horizontal-line test to determine whether the inverse of each relation is a function.

The inverse is a function because no horizontal line passes through two points on the graph.

Example 3


To write the rule for the inverse of a function, you can exchange x and y and solve the equation for y. Because the value of x and y are switched, the domain of the function will be the range of its inverse and vice versa.

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context. Example 4

Find the inverse of . Determine whether it is a function, and state its domain and range.

Rewrite the function using y instead of f(x).

Step 1 Find the inverse.

Simplify.

Switch x and y in the equation.

Cube both sides.

Isolate y.

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context. Example 4 Continued

The inverse is a function, . B/c?

The domain of the inverse is the range of f(x):{x|x R}.

The range is the domain of f(x):{y|y R}.Check Graph both relations to see that they are

symmetric about y = x.


Rewrite the function using y instead of f(x).

Step 1 Find the inverse.

Take the cube root of both sides.

Switch x and y in the equation.

Add 2 to both sides of the equation.

Simplify.

Example 5

y = x3 – 2

x = y3 – 2

x + 2 = y3

3 x + 2 = y

33 3x + 2 = y

Find the inverse of f(x) = x3 – 2. Determine whether it is a function, and state its domain and range.


The domain of the inverse is the range of f(x): R.

The range is the domain of f(x): R.

Check Graph both relations to see that they are symmetric about y = x.

Example 5 Continued

Because the inverse is a function, .


You have seen that the inverses of functions are not necessarily functions. When both a relation and its inverses are functions, the relation is called a one-to-one function. In a one-to-one function, each y-value is paired with exactly one x-value.

You can use composition of functions to verify that two functions are inverses. Because inverse functions “undo” each other, when you compose two inverses the result is the input value x.


Determine by composition whether each pair of functions are inverses.

Example 6

Find the composition f(g(x)).

f(g(x)) = 3( x + 1) – 1 1 3

Use the Distributive Property.

Simplify.

f(x) = 3x – 1 and g(x) = x + 1 1 3

Substitute x + 1 for x in f.

1 3

= (x + 3) – 1

= x + 2


Because f(g(x)) ≠ x, f and g are not inverses. There is no need to check g(f(x)).

Example 6 Continued

Check The graphs are not symmetric about the line y = x.

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context. Example 7

Find the compositions f(g(x)) and g(f (x)).

For x ≠ 1 or 0, f(x) = and g(x) = + 1. 1 x

1x – 1

Because f(g(x)) = g(f (x)) = x for all x but 0 and 1, f and g are inverses.

= x

= (x – 1) + 1

= x

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context. Example 7 Continued

Check The graphs are symmetric about the line y = x for all x but 0 and 1.


Independent Practice

Due Tomorrow at Beginning of Class

p. 453-454 # 9-21, 25-35 odd

5.4 Functions and Their InversesCC.9-12.F.BF.1c (+) Compose functions. CC.9-12.A.CED.2 Create equations in two or more variables to represent relationships between quantities; graph equations on coordinate axes with labels and scales. CC.9-12.A.CED.3 Represent constraints by equations or inequalities, and by systems of equations and/or inequalities, and interpret solutions as viable or nonviable options in a modeling context. Lesson Quiz: Part I

A: yes; B: no

1. Use the horizontal-line test to determine whether the inverse of each relation is a function.


Lesson Quiz: Part II

D: {x|x ≥ 4}; R: {all Real Numbers}

2. Find the inverse f(x) = x2 – 4. Determine whether it is a function, and state its domain and range.

not a function


3. Determine by composition whether f(x) =

3(x – 1)2 and g(x) = +1 are inverses

for x ≥ 0.

Lesson Quiz: Part III

yes

5.4 functions and their inverses cc.9-12.f.bf.1c (+) compose functions. cc.9-12.a.ced.2 create...

Documents

quantities graph equations

inverses of functions

coordinate axes

nonviable options

modeling context

function fx

line y

horizontalline test