<< Chapter < Page Chapter >> Page >

Composition of functions

When the output of one function is used as the input of another, we call the entire operation a composition of functions. For any input x and functions f and g , this action defines a composite function    , which we write as f g such that

( f g ) ( x ) = f ( g ( x ) )

The domain of the composite function f g is all x such that x is in the domain of g and g ( x ) is in the domain of f .

It is important to realize that the product of functions f g is not the same as the function composition f ( g ( x ) ) , because, in general, f ( x ) g ( x ) f ( g ( x ) ) .

Determining whether composition of functions is commutative

Using the functions provided, find f ( g ( x ) ) and g ( f ( x ) ) . Determine whether the composition of the functions is commutative .

f ( x ) = 2 x + 1 g ( x ) = 3 x

Let’s begin by substituting g ( x ) into f ( x ) .

  f ( g ( x ) ) = 2 ( 3 x ) + 1                 = 6 2 x + 1                 = 7 2 x

Now we can substitute f ( x ) into g ( x ) .

  g ( f ( x ) ) = 3 ( 2 x + 1 )                 = 3 2 x 1                 = 2 x + 2

We find that g ( f ( x ) ) f ( g ( x ) ) , so the operation of function composition is not commutative.

Got questions? Get instant answers now!
Got questions? Get instant answers now!

Interpreting composite functions

The function c ( s ) gives the number of calories burned completing s sit-ups, and s ( t ) gives the number of sit-ups a person can complete in t minutes. Interpret c ( s ( 3 ) ) .

The inside expression in the composition is s ( 3 ) . Because the input to the s -function is time, t = 3 represents 3 minutes, and s ( 3 ) is the number of sit-ups completed in 3 minutes.

Using s ( 3 ) as the input to the function c ( s ) gives us the number of calories burned during the number of sit-ups that can be completed in 3 minutes, or simply the number of calories burned in 3 minutes (by doing sit-ups).

Got questions? Get instant answers now!
Got questions? Get instant answers now!

Investigating the order of function composition

Suppose f ( x ) gives miles that can be driven in x hours and g ( y ) gives the gallons of gas used in driving y miles. Which of these expressions is meaningful: f ( g ( y ) ) or g ( f ( x ) ) ?

The function y = f ( x ) is a function whose output is the number of miles driven corresponding to the number of hours driven.

number of miles  = f ( number of hours )

The function g ( y ) is a function whose output is the number of gallons used corresponding to the number of miles driven. This means:

number of gallons  = g ( number of miles )

The expression g ( y ) takes miles as the input and a number of gallons as the output. The function f ( x ) requires a number of hours as the input. Trying to input a number of gallons does not make sense. The expression f ( g ( y ) ) is meaningless.

The expression f ( x ) takes hours as input and a number of miles driven as the output. The function g ( y ) requires a number of miles as the input. Using f ( x ) (miles driven) as an input value for g ( y ) , where gallons of gas depends on miles driven, does make sense. The expression g ( f ( x ) ) makes sense, and will yield the number of gallons of gas used, g , driving a certain number of miles, f ( x ) , in x hours.

Got questions? Get instant answers now!
Got questions? Get instant answers now!

Are there any situations where f ( g ( y ) ) And g ( f ( x ) ) Would both be meaningful or useful expressions?

Yes. For many pure mathematical functions, both compositions make sense, even though they usually produce different new functions. In real-world problems, functions whose inputs and outputs have the same units also may give compositions that are meaningful in either order.

Questions & Answers

what is biology
Hajah Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
what is biology
Victoria Reply
HOW CAN MAN ORGAN FUNCTION
Alfred Reply
the diagram of the digestive system
Assiatu Reply
allimentary cannel
Ogenrwot
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
Hassan Reply
discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
what is biology
Yousuf Reply
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply
Practice Key Terms 1

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Precalculus' conversation and receive update notifications?

Ask