audio read-through Graphs of Functions

To prepare for this section, you may find it helpful to review the following concepts in the Algebra I curriculum:

Recall that a function is a rule that takes an input, does something to it, and gives an output. Each input has exactly one output.

If the function name is $\,f\,,$ and the input name is $\,x\,,$ then the unique corresponding output is called $\,f(x)\,$ (which is read aloud as ‘$\,f\,$ of $\,x\,$’). See function notation for details.

When you're working with a function, it's critical that you understand the relationship between its inputs and their corresponding outputs. That is, it's critical that you understand the function's (input,output) pairs. Of course, there are usually infinitely many of these (input,output) pairs—the graph of the function!

For example, consider the squaring function—the function that takes a real number input, and squares it.

When the input is $\,3\,,$ the output is $\,3^2 = 9\,.$ Thus, $\,(3,9)\,$ is an (input,output) pair.

When the input is $\,4\,,$ the output is $\,4^2 = 16\,.$ Thus, $\,(4,16)\,$ is an (input,output) pair.

When the input is $\,-3\,,$ the output is $\,(-3)^2 = 9\,.$ Thus, $\,(-3,9)\,$ is an (input,output) pair.

Here's a table (below) that summarizes a few of the infinitely-many (input,output) pairs. Of course, it's impossible to list them all.

When these points are plotted in an $\,xy\,$-coordinate system (see below), with the inputs along the $\,x\,$-axis and the outputs along the $\,y\,$-axis, a shape clearly emerges in the coordinate plane.

Some (Input,Output) Pairs for the Squaring Function

input output (input,output)
$-3$ $9$ $(-3,9)$
$-2$ $4$ $(-2,4)$
$-1$ $1$ $(-1,1)$
$0$ $0$ $(0,0)$
$\frac12$ $\frac14$ $(\frac12,\frac14)$
$1$ $1$ $(1,1)$
$2.3$ $5.29$ $(2.3,5.29)$
$\pi$ $\pi^2$ $(\pi,\pi^2)$
graph of the squaring function

The picture of all the points of the form $\,(x,x^2)\,$ is called the graph of the squaring function.

Now it's time to make things precise:

DEFINITION graph of a function
Let $\,f\,$ be a function with domain $\,\text{dom}(f)\,.$

The graph of $\,f\,$ is the picture of all its (input,output) pairs.

Precisely:

$$ \cssId{s42}{\text{graph of } f = \{(x,f(x))\ |\ x\in\text{dom}(f)\} } $$

(Read this aloud as: ‘The graph of $\,f\,$ is the set of all points of the form $\,x\,,$ comma, $\,f\,$ of $\,x\,,$ with the property that $\,x\,$ is in the domain of $\,f\,$.’)

When you graph a function:

  • the inputs (the first coordinates of the points) are placed along the $\,x$-axis;
  • the outputs (the second coordinates of the points) are placed along the $\,y$-axis.

The graph itself should then be labeled $\,y=f(x)\,$; this indicates that the $\,y$-value of each point is the output from the function $\,f\,$ when the input is $\,x\,.$

Different names (other than $\,x\,$ and $\,y\,$) may certainly be used for the inputs and outputs; the graph should be labeled accordingly.

The sketch below illustrates all the key features of a graph.

A couple specific (input,output) pairs are shown.

graph of the squaring function

You should recognize this as the graph of the squaring function. That is, $\,f(x) = x^2\,.$ Thus:

$$\begin{gather} f(-2) = (-2)^2 = 4\cr \text{and}\cr f(1) = 1^2 = 1 \end{gather} $$

Alternative names for inputs and outputs have been chosen for the graph below:

graph of the cubing function

A couple specific (input,output) pairs are shown.

You may have guessed that this is the graph of the cubing function. That is, $\,g(t) = t^3\,.$ Thus:

$$ \begin{gather} \cssId{s72}{g(-1) = (-1)^3 = -1}\cr \text{and}\cr \cssId{s73}{g(2) = 2^3 = 8} \end{gather} $$

Two Ways to Ask the Same Question

Here are two things you will frequently be asked to do:

Two different-sounding questions, but exactly the same answer. It's very important that you are comfortable with these interchangeable ways that you might be asked for a graph.

Example: Reading Information From a Graph

The graph of a function $\,f\,$ is shown below.

reading information from a graph

Read the following information from the graph:

Solutions

Concept Practice