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# Hawkes Learning Systems: College Algebra - PowerPoint PPT Presentation

Hawkes Learning Systems: College Algebra. Section 4.5 : Combining Functions. Objectives. Combining functions arithmetically. Composing functions. Decomposing functions. Combining Functions Arithmetically. Addition, Subtraction, Multiplication and Division of Functions 1. 2. 3. 4.

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### Hawkes Learning Systems:College Algebra

Section 4.5: Combining Functions

• Combining functions arithmetically.

• Composing functions.

• Decomposing functions.

Addition, Subtraction, Multiplication and Division of Functions

1.

2.

3.

4.

The domain of each of these new functions consists of the common elements (or the intersection of elements) of the domains of f and g individually.

Given that solve:

a.

Remember that

Continued on the next slide…

Given that solve:

b.

Remember that

Given that find a. and b.

a.

b.

Remember that

Remember that

Composing Functions

Let f and g be two functions. The composition of f and g, denoted , is the function defined by .

The domain of consists of all x in the domain of g for which g(x) is in turn in the domain of f. The function is read “f composed with g,” or “f of g.”

Caution!

Note that the order of f and g is important. In general, we can expect the function to be different from the function . In formal terms, the composition of two functions, unlike the sum and product of two functions, is not commutative.

The diagram below is a schematic of the composition of two functions. The ovals represent sets, with the leftmost oval being the domain of the function g. The arrows indicate the element that x is associated with by the various functions.

Given f(x) = x2and g(x) = x + 5 , find:

a.

= 112

= 121

First, we will find g(6) by replacing x with 6 in g(x).

Next, we know that f composed with g can also be written . Since we already evaluated g(6), we can insert the answer to get f(11).

Continued on the next slide…

Given f(x) = x2+ 2 and g(x) = x + 5 , find:

b.

= (x + 5)2 + 2

= x2 +10x + 25 + 2

= x2 +10x + 27

Again, we know by definition that .

Note: since we solved for the variable x we should be able to plug 6 into x and get the same answer as in part a. Verify this.

Let f(x) = x – 6 and g(x) = . Simplify the composition and find the domain for:

= g(x – 6)

=

Domain: [6, )

The domain of must be any x such that x – 6 > 0 since x - 6 is under a radical.

Often functions can be best understood by recognizing them as a composition of two or more simpler functions. For example, the function can be thought of as the composition of two or more functions.

Note: if then:

Ex: The function can be written as a composition of functions in many different ways. Some of the decompositions of f(x) are shownbelow:

a.

b.

c.

Decompose the function into:

a. a composition of two functions

b. a composition of three functions

Note: These are NOTthe only possible solutions for the decompositions of f(x)!