Chapter 6 6-5 operations with functions

ObjectivesAdd, subtract, multiply, and divide

functions. Write and evaluate composite functions.

Operations with functions You can perform operations on functions

in much the same way that you perform operations on numbers or expressions. You can add, subtract, multiply, or divide functions by operating on their rules.

Notation

Example 1A: Adding and Subtracting Functions

Given f(x) = 4x2 + 3x – 1 and g(x) = 6x + 2, find each function.

(f + g)(x) Solution: (f + g)(x) = f(x) + g(x)

= (4x2 + 3x – 1) + (6x + 2)

= 4x2 + 9x + 1

Example 1B: Adding and Subtracting Functions

Given f(x) = 4x2 + 3x – 1 and g(x) = 6x + 2, find each function.

Solution: (f – g)(x) = f(x) – g(x) = (4x2 + 3x – 1) – (6x + 2) = 4x2 + 3x – 1 – 6x – 2

(f – g)(x)

= 4x2 – 3x – 3

Check It Out! Example 1a

Given f(x) = 5x – 6 and g(x) = x2 – 5x + 6, find each function.

(f + g)(x) = x2

(f – g)(x) = –x2 + 10x – 12

When you divide functions, be sure to note any domain restrictions that may arise.

Example 2A: Multiplying and Dividing Functions

Given f(x) = 6x2 – x – 12 and g(x) = 2x – 3, find each function.

(fg)(x) Solution (fg)(x) = f(x) ● g(x) = (6x2 – x – 12) (2x – 3) 6x2 (2x – 3) – x(2x – 3) – 12(2x – 3) 12x3 – 18x2 – 2x2 + 3x – 24x + 36 = 12x3 – 20x2 – 21x + 36

Example 2B: Multiplying and Dividing Functions

Given f(x) = 6x2 – x – 12 and g(x) = 2x – 3, find each function

( f/g )(x)

Solution

( ) (x)fg

f(x)

g(x)=

6x2 – x –12

2x – 3=

=(2x – 3)(3x + 4)

2x – 3

=(2x – 3)(3x +4)

(2x – 3)

3x + 4, where x ≠ 3/2

Check it out Given f(x) = x + 2 and g(x) = x2 – 4,

find each function. (fg)(x)

= x – 2, where x ≠ –2

= x3 + 2x2 – 4x – 8

( ) (x)g

f

Composition of functions Another function operation uses the

output from one function as the input for a second function. This operation is called the composition of functions.

Composition of function

The order of function operations is the same as the order of operations for numbers and expressions. To find f(g(3)), evaluate g(3) first and then substitute the result into f.

Example 3A: Evaluating Composite Functions

Given f(x) = 2x and g(x) = 7 – x, find each value.

Solution:Step 1 Find g(4)

f(g(4))

g(4) = 7 – 4 = 3

Step 2 Find f(3)

f(3) = 23 = 8

So f(g(4)) = 8.

Example Given f(x) = 2x and g(x) = 7 – x, find

each value.

g(f(4))

Check It Out! Example 3a

Given f(x) = 2x – 3 and g(x) = x2, find each value.

f(g(3)) So f(g(3)) = 15.

g(f(3))So g(f(3)) = 9

Operations with functions You can use algebraic expressions as

well as numbers as inputs into functions. To find a rule for f(g(x)), substitute the rule for g into f.

Example 4A: Writing Composite Functions

Given f(x) = x2 – 1 and g(x) = , write each composite function. State the domain of each.

f(g(x)) Solution

f(g(x)) = f( x

1 – x

= ( )2 – 1 x

1 – x

–1 + 2x

(1 – x)2=

The domain of f(g(x)) is x ≠ 1 or {x|x ≠ 1} because g(1) is undefined.

Example Given f(x) = x2 – 1 and g(x) = ,

write each composite function. State the domain of each.

g(f(x))

The domain of g(f(x)) is x ≠ or {x|x ≠ } because f( ) = 1 and g(1) is undefined.

Check It Out! Example 4a

Given f(x) = 3x – 4 and g(x) = + 2 , write each composite. State the domain of each.

f(g(x))

The domain of f(g(x)) is x ≥ 0 or {x|x ≥ 0}.

g(f(x))

Composite functions Composite functions can be used to

simplify a series of functions.

Application Jake imports furniture from Mexico.

The exchange rate is 11.30 pesos per U.S. dollar. The cost of each piece of furniture is given in pesos. The total cost of each piece of furniture includes a 15% service charge.

A. Write a composite function to represent the total cost of a piece of furniture in dollars if the cost of the item is c pesos.

Solution Step 1 Write a function for the total

cost in U.S. dollars.

P(c) = c + 0.15c

= 1.15c

Step 2 Write a function for the cost in dollars based on the cost in pesos.

D(c) = c

11.30

Solutiob Step 3 Find the composition D(P(c)).

= 1.15 ( ) c

11.30

B. Find the total cost of a table in dollars if it costs 1800 pesos.

Evaluate the composite function for c = 1800.

D(P(c) ) = 1.15 ( )1800

11.30

≈ 183.19

Student guided practice Do even numbers from 2-14

Homework Do even numbers 15-32 in your book

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