# 1.4 Composition of functions  (Page 2/9)

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## Performing algebraic operations on functions

Find and simplify the functions $\text{\hspace{0.17em}}\left(g-f\right)\left(x\right)\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}\left(\frac{g}{f}\right)\left(x\right),\text{\hspace{0.17em}}$ given $\text{\hspace{0.17em}}f\left(x\right)=x-1\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\left(x\right)={x}^{2}-1.\text{\hspace{0.17em}}$ Are they the same function?

Begin by writing the general form, and then substitute the given functions.

No, the functions are not the same.

Note: For $\text{\hspace{0.17em}}\left(\frac{g}{f}\right)\left(x\right),\text{\hspace{0.17em}}$ the condition $\text{\hspace{0.17em}}x\ne 1\text{\hspace{0.17em}}$ is necessary because when $\text{\hspace{0.17em}}x=1,\text{\hspace{0.17em}}$ the denominator is equal to 0, which makes the function undefined.

Find and simplify the functions $\text{\hspace{0.17em}}\left(fg\right)\left(x\right)\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}\left(f-g\right)\left(x\right).$

Are they the same function?

$\begin{array}{l}\left(fg\right)\left(x\right)=f\left(x\right)g\left(x\right)=\left(x-1\right)\left({x}^{2}-1\right)={x}^{3}-{x}^{2}-x+1\\ \left(f-g\right)\left(x\right)=f\left(x\right)-g\left(x\right)=\left(x-1\right)-\left({x}^{2}-1\right)=x-{x}^{2}\end{array}$

No, the functions are not the same.

## Create a function by composition of functions

Performing algebraic operations on functions combines them into a new function, but we can also create functions by composing functions. When we wanted to compute a heating cost from a day of the year, we created a new function that takes a day as input and yields a cost as output. The process of combining functions so that the output of one function becomes the input of another is known as a composition of functions . The resulting function is known as a composite function . We represent this combination by the following notation:

$\left(f\circ g\right)\left(x\right)=f\left(g\left(x\right)\right)$

We read the left-hand side as $“f\text{\hspace{0.17em}}$ composed with $\text{\hspace{0.17em}}g\text{\hspace{0.17em}}$ at $\text{\hspace{0.17em}}x,”$ and the right-hand side as $“f\text{\hspace{0.17em}}$ of $\text{\hspace{0.17em}}g\text{\hspace{0.17em}}$ of $\text{\hspace{0.17em}}x.”$ The two sides of the equation have the same mathematical meaning and are equal. The open circle symbol $\text{\hspace{0.17em}}\circ \text{\hspace{0.17em}}$ is called the composition operator. We use this operator mainly when we wish to emphasize the relationship between the functions themselves without referring to any particular input value. Composition is a binary operation that takes two functions and forms a new function, much as addition or multiplication takes two numbers and gives a new number. However, it is important not to confuse function composition with multiplication because, as we learned above, in most cases $f\left(g\left(x\right)\right)\ne f\left(x\right)g\left(x\right).$

It is also important to understand the order of operations in evaluating a composite function. We follow the usual convention with parentheses by starting with the innermost parentheses first, and then working to the outside. In the equation above, the function $\text{\hspace{0.17em}}g\text{\hspace{0.17em}}$ takes the input $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ first and yields an output $\text{\hspace{0.17em}}g\left(x\right).\text{\hspace{0.17em}}$ Then the function $\text{\hspace{0.17em}}f\text{\hspace{0.17em}}$ takes $\text{\hspace{0.17em}}g\left(x\right)\text{\hspace{0.17em}}$ as an input and yields an output $\text{\hspace{0.17em}}f\left(g\left(x\right)\right).$

In general, $\text{\hspace{0.17em}}f\circ g\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\circ f\text{\hspace{0.17em}}$ are different functions. In other words, in many cases $\text{\hspace{0.17em}}f\left(g\left(x\right)\right)\ne g\left(f\left(x\right)\right)\text{\hspace{0.17em}}$ for all $\text{\hspace{0.17em}}x.\text{\hspace{0.17em}}$ We will also see that sometimes two functions can be composed only in one specific order.

For example, if $\text{\hspace{0.17em}}f\left(x\right)={x}^{2}\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\left(x\right)=x+2,$ then

but

These expressions are not equal for all values of $\text{\hspace{0.17em}}x,\text{\hspace{0.17em}}$ so the two functions are not equal. It is irrelevant that the expressions happen to be equal for the single input value $\text{\hspace{0.17em}}x=-\frac{1}{2}.$

Note that the range of the inside function (the first function to be evaluated) needs to be within the domain of the outside function. Less formally, the composition has to make sense in terms of inputs and outputs.

The center is at (3,4) a focus is at (3,-1), and the lenght of the major axis is 26
The center is at (3,4) a focus is at (3,-1) and the lenght of the major axis is 26 what will be the answer?
Rima
I done know
Joe
What kind of answer is that😑?
Rima
I had just woken up when i got this message
Joe
Rima
i have a question.
Abdul
how do you find the real and complex roots of a polynomial?
Abdul
@abdul with delta maybe which is b(square)-4ac=result then the 1st root -b-radical delta over 2a and the 2nd root -b+radical delta over 2a. I am not sure if this was your question but check it up
Nare
This is the actual question: Find all roots(real and complex) of the polynomial f(x)=6x^3 + x^2 - 4x + 1
Abdul
@Nare please let me know if you can solve it.
Abdul
I have a question
juweeriya
hello guys I'm new here? will you happy with me
mustapha
The average annual population increase of a pack of wolves is 25.
how do you find the period of a sine graph
Period =2π if there is a coefficient (b), just divide the coefficient by 2π to get the new period
Am
if not then how would I find it from a graph
Imani
by looking at the graph, find the distance between two consecutive maximum points (the highest points of the wave). so if the top of one wave is at point A (1,2) and the next top of the wave is at point B (6,2), then the period is 5, the difference of the x-coordinates.
Am
you could also do it with two consecutive minimum points or x-intercepts
Am
I will try that thank u
Imani
Case of Equilateral Hyperbola
ok
Zander
ok
Shella
f(x)=4x+2, find f(3)
Benetta
f(3)=4(3)+2 f(3)=14
lamoussa
14
Vedant
pre calc teacher: "Plug in Plug in...smell's good" f(x)=14
Devante
8x=40
Chris
Explain why log a x is not defined for a < 0
the sum of any two linear polynomial is what
Momo
how can are find the domain and range of a relations
the range is twice of the natural number which is the domain
Morolake
A cell phone company offers two plans for minutes. Plan A: $15 per month and$2 for every 300 texts. Plan B: $25 per month and$0.50 for every 100 texts. How many texts would you need to send per month for plan B to save you money?
6000
Robert
more than 6000
Robert
can I see the picture
How would you find if a radical function is one to one?
how to understand calculus?
with doing calculus
SLIMANE
Thanks po.
Jenica
Hey I am new to precalculus, and wanted clarification please on what sine is as I am floored by the terms in this app? I don't mean to sound stupid but I have only completed up to college algebra.
I don't know if you are looking for a deeper answer or not, but the sine of an angle in a right triangle is the length of the opposite side to the angle in question divided by the length of the hypotenuse of said triangle.
Marco
can you give me sir tips to quickly understand precalculus. Im new too in that topic. Thanks
Jenica
if you remember sine, cosine, and tangent from geometry, all the relationships are the same but they use x y and r instead (x is adjacent, y is opposite, and r is hypotenuse).
Natalie
it is better to use unit circle than triangle .triangle is only used for acute angles but you can begin with. Download any application named"unit circle" you find in it all you need. unit circle is a circle centred at origine (0;0) with radius r= 1.
SLIMANE
What is domain
johnphilip
the standard equation of the ellipse that has vertices (0,-4)&(0,4) and foci (0, -15)&(0,15) it's standard equation is x^2 + y^2/16 =1 tell my why is it only x^2? why is there no a^2?
what is foci?
This term is plural for a focus, it is used for conic sections. For more detail or other math questions. I recommend researching on "Khan academy" or watching "The Organic Chemistry Tutor" YouTube channel.
Chris