# 1.4 Composition of functions  (Page 4/9)

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The gravitational force on a planet a distance r from the sun is given by the function $G\left(r\right).$ The acceleration of a planet subjected to any force $F$ is given by the function $a\left(F\right).$ Form a meaningful composition of these two functions, and explain what it means.

A gravitational force is still a force, so $a\left(G\left(r\right)\right)$ makes sense as the acceleration of a planet at a distance r from the Sun (due to gravity), but $G\left(a\left(F\right)\right)$ does not make sense.

## Evaluating composite functions

Once we compose a new function from two existing functions, we need to be able to evaluate it for any input in its domain. We will do this with specific numerical inputs for functions expressed as tables, graphs, and formulas and with variables as inputs to functions expressed as formulas. In each case, we evaluate the inner function using the starting input and then use the inner function’s output as the input for the outer function.

## Evaluating composite functions using tables

When working with functions given as tables, we read input and output values from the table entries and always work from the inside to the outside. We evaluate the inside function first and then use the output of the inside function as the input to the outside function.

## Using a table to evaluate a composite function

Using [link] , evaluate $\text{\hspace{0.17em}}f\left(g\left(3\right)\right)\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\left(f\left(3\right)\right).$

$x$ $f\left(x\right)$ $g\left(x\right)$
1 6 3
2 8 5
3 3 2
4 1 7

To evaluate $\text{\hspace{0.17em}}f\left(g\left(3\right)\right),\text{\hspace{0.17em}}$ we start from the inside with the input value 3. We then evaluate the inside expression $\text{\hspace{0.17em}}g\left(3\right)\text{\hspace{0.17em}}$ using the table that defines the function $\text{\hspace{0.17em}}g:\text{\hspace{0.17em}}$ $g\left(3\right)=2.\text{\hspace{0.17em}}$ We can then use that result as the input to the function $\text{\hspace{0.17em}}f,\text{\hspace{0.17em}}$ so $\text{\hspace{0.17em}}g\left(3\right)\text{\hspace{0.17em}}$ is replaced by 2 and we get $\text{\hspace{0.17em}}f\left(2\right).\text{\hspace{0.17em}}$ Then, using the table that defines the function $\text{\hspace{0.17em}}f,\text{\hspace{0.17em}}$ we find that $\text{\hspace{0.17em}}f\left(2\right)=8.$

$\begin{array}{l}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}g\left(3\right)=2\hfill \\ \text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}f\left(g\left(3\right)\right)=f\left(2\right)=8\hfill \end{array}$

To evaluate $\text{\hspace{0.17em}}g\left(f\left(3\right)\right),\text{\hspace{0.17em}}$ we first evaluate the inside expression $\text{\hspace{0.17em}}f\left(3\right)\text{\hspace{0.17em}}$ using the first table: $\text{\hspace{0.17em}}f\left(3\right)=3.\text{\hspace{0.17em}}$ Then, using the table for $\text{\hspace{0.17em}}g\text{,\hspace{0.17em}}$ we can evaluate

$g\left(f\left(3\right)\right)=g\left(3\right)=2$

[link] shows the composite functions $\text{\hspace{0.17em}}f\circ g\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\circ f\text{\hspace{0.17em}}$ as tables.

 $x$ $g\left(x\right)$ $f\left(g\left(x\right)\right)$ $f\left(x\right)$ $g\left(f\left(x\right)\right)$ 3 2 8 3 2

Using [link] , evaluate $\text{\hspace{0.17em}}f\left(g\left(1\right)\right)\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\left(f\left(4\right)\right).$

$f\left(g\left(1\right)\right)=f\left(3\right)=3\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}g\left(f\left(4\right)\right)=g\left(1\right)=3$

## Evaluating composite functions using graphs

When we are given individual functions as graphs, the procedure for evaluating composite functions is similar to the process we use for evaluating tables. We read the input and output values, but this time, from the $\text{\hspace{0.17em}}x\text{-}$ and $y\text{-}$ axes of the graphs.

Given a composite function and graphs of its individual functions, evaluate it using the information provided by the graphs.

1. Locate the given input to the inner function on the $\text{\hspace{0.17em}}x\text{-}$ axis of its graph.
2. Read off the output of the inner function from the $\text{\hspace{0.17em}}y\text{-}$ axis of its graph.
3. Locate the inner function output on the $\text{\hspace{0.17em}}x\text{-}$ axis of the graph of the outer function.
4. Read the output of the outer function from the $\text{\hspace{0.17em}}y\text{-}$ axis of its graph. This is the output of the composite function.

## Using a graph to evaluate a composite function

Using [link] , evaluate $\text{\hspace{0.17em}}f\left(g\left(1\right)\right).$

To evaluate $\text{\hspace{0.17em}}f\left(g\left(1\right)\right),\text{\hspace{0.17em}}$ we start with the inside evaluation. See [link] .

We evaluate $\text{\hspace{0.17em}}g\left(1\right)\text{\hspace{0.17em}}$ using the graph of $\text{\hspace{0.17em}}g\left(x\right),\text{\hspace{0.17em}}$ finding the input of 1 on the $\text{\hspace{0.17em}}x\text{-}$ axis and finding the output value of the graph at that input. Here, $\text{\hspace{0.17em}}g\left(1\right)=3.\text{\hspace{0.17em}}$ We use this value as the input to the function $\text{\hspace{0.17em}}f.$

$f\left(g\left(1\right)\right)=f\left(3\right)$

We can then evaluate the composite function by looking to the graph of $\text{\hspace{0.17em}}f\left(x\right),\text{\hspace{0.17em}}$ finding the input of 3 on the $x\text{-}$ axis and reading the output value of the graph at this input. Here, $\text{\hspace{0.17em}}f\left(3\right)=6,\text{\hspace{0.17em}}$ so $\text{\hspace{0.17em}}f\left(g\left(1\right)\right)=6.$

"7"has an open circle and "10"has a filled in circle who can I have a set builder notation
x=-b+_Гb2-(4ac) ______________ 2a
I've run into this: x = r*cos(angle1 + angle2) Which expands to: x = r(cos(angle1)*cos(angle2) - sin(angle1)*sin(angle2)) The r value confuses me here, because distributing it makes: (r*cos(angle2))(cos(angle1) - (r*sin(angle2))(sin(angle1)) How does this make sense? Why does the r distribute once
so good
abdikarin
this is an identity when 2 adding two angles within a cosine. it's called the cosine sum formula. there is also a different formula when cosine has an angle minus another angle it's called the sum and difference formulas and they are under any list of trig identities
strategies to form the general term
carlmark
How can you tell what type of parent function a graph is ?
generally by how the graph looks and understanding what the base parent functions look like and perform on a graph
William
if you have a graphed line, you can have an idea by how the directions of the line turns, i.e. negative, positive, zero
William
y=x will obviously be a straight line with a zero slope
William
y=x^2 will have a parabolic line opening to positive infinity on both sides of the y axis vice versa with y=-x^2 you'll have both ends of the parabolic line pointing downward heading to negative infinity on both sides of the y axis
William
y=x will be a straight line, but it will have a slope of one. Remember, if y=1 then x=1, so for every unit you rise you move over positively one unit. To get a straight line with a slope of 0, set y=1 or any integer.
Aaron
yes, correction on my end, I meant slope of 1 instead of slope of 0
William
what is f(x)=
I don't understand
Joe
Typically a function 'f' will take 'x' as input, and produce 'y' as output. As 'f(x)=y'. According to Google, "The range of a function is the complete set of all possible resulting values of the dependent variable (y, usually), after we have substituted the domain."
Thomas
Sorry, I don't know where the "Â"s came from. They shouldn't be there. Just ignore them. :-)
Thomas
Darius
Thanks.
Thomas
Â
Thomas
It is the Â that should not be there. It doesn't seem to show if encloses in quotation marks. "Â" or 'Â' ... Â
Thomas
Now it shows, go figure?
Thomas
what is this?
i do not understand anything
unknown
lol...it gets better
Darius
I've been struggling so much through all of this. my final is in four weeks 😭
Tiffany
this book is an excellent resource! have you guys ever looked at the online tutoring? there's one that is called "That Tutor Guy" and he goes over a lot of the concepts
Darius
thank you I have heard of him. I should check him out.
Tiffany
is there any question in particular?
Joe
I have always struggled with math. I get lost really easy, if you have any advice for that, it would help tremendously.
Tiffany
Sure, are you in high school or college?
Darius
Hi, apologies for the delayed response. I'm in college.
Tiffany
how to solve polynomial using a calculator
So a horizontal compression by factor of 1/2 is the same as a horizontal stretch by a factor of 2, right?
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