Let’s Think About It
The championship bicycling race is a major event. Jess has trained for years to compete at this level and can go quite fast. Starting from rest, Jess can accelerate to 20 miles per hour. The coach records times and speeds and puts them in a table (shown below). The coach wants to measure Jess’s progress and to see if there is a pattern in this data.
Input (time)  Output (speed in mph) 
0  0 
1  2 
3  5 
5  10 
8  20 
Can you plot this data on a coordinate plane, and then determine if this data could be modeled by a linear function?
In this concept, you will learn to distinguish between linear and nonlinear functions.
Guidance
If you graph a linear function you will get a straight line. There are also nonlinear functions. If you graph the coordinates of a nonlinear function you will not get a straight line.
One of the easiest ways (but not the only way) to distinguish between a linear and a nonlinear function is to look at the graph of the function. Look at the two graphs below and you will see the difference between the two types of functions.
The first graph above is a linear function because its graph is a straight line. The second graph is a nonlinear function. Notice that the graph of this function is not a straight line. It is curved.
So, if you plot points from a function and cannot draw a straight line through them, then it is not a linear function.
Let’s look at an example.
The equation
is a function. Plot several points to sketch a graph of the function.
 Determine if the function linear or nonlinear.
First, consider part a.
Use the equation to create an inputoutput table. Use five
values so you get a sense of what the function will look like when graphed. Pick positive and negative values centered around 0.Input \begin{align*}(x)\end{align*} 
Output  
\begin{align*}2\end{align*} 
\begin{align*}4\end{align*} 
\begin{align*}y = (2)^2 = (2) \cdot (2) = 4\end{align*} 
\begin{align*}1\end{align*} 
\begin{align*}y = (1)^2 = (1) \cdot (1) = 1\end{align*} 

\begin{align*}0\end{align*} 
\begin{align*}0\end{align*} 
\begin{align*}y = (0)^2 = (0) \cdot (0) = 0\end{align*} 
\begin{align*}1\end{align*} 
\begin{align*}1\end{align*} 
\begin{align*}y = (1)^2 = (1) \cdot (1) = 1\end{align*} 
\begin{align*}2\end{align*} 
\begin{align*}4\end{align*} 
\begin{align*}y = (2)^2 = (2) \cdot (2) = 4\end{align*} 
The ordered pairs shown in the table are
and .Plot those five points on the coordinate plane. Then, draw a curve to connect them.
The graph of
is shown below. This kind of curve is called a parabola.
Next, consider part b.
Notice that you cannot connect these points with a straight line. You will need to draw a curved line to connect them.
The answer is
is not a linear function.Guided Practice
Look at the graph of a function below. Is this a linear or nonlinear function? Explain your answer. Then, write the coordinates of the ordered pairs highlighted on the graph.
The answer is that this is a linear function because the graph is a straight line.
The ordered pairs graphed are
.Examples
Several points from a function have been provided in the examples below. Determine if the points could be part of a linear function.
Example 1
Plot these points on a coordinate plane. Since you can connect them with straight line (using a straight edge), they could be part of a linear function.
The answer is these points could be from a linear function.
Example 2
Plot these points on a coordinate plane. Because you cannot connect these lines with a straight line, these points are not part of a linear function.
The answer is these points are from a nonlinear function.
Example 3
Plot these points on a coordinate plane. Since you can connect them with straight line (using a straight edge), they could be part of a linear function.
The answer is these points could be from a linear function.
Follow Up
Remember the bike race?
Jess’s coach recorded times and speeds in the table below. You need to determine if this data could be modeled by a linear function.
Input (time)  Output (speed in mph) 
0  0 
1  2 
3  5 
5  10 
8  20 
First, write the information in this table as coordinates:
.Then, plot these points (shown below).
Next, determine if a linear function could model this data. Can you draw a straight (not curved) line that connects all of these points? You can get close, but a straight line will not go through all of the points, and is likely not the best model for this data.
The answer is no, this data is not part of a linear function.
Video Review
https://www.youtube.com/watch?v=8KLDGlrjzaw&feature=youtu.be
Explore More
Determine if each graph shows a linear function or a nonlinear function.
1.
2.
3.
4. The equation
is a function. Complete the table below to identify five ordered pairs for this function, and then plot the points on a coordinate plane. Then connect those points to sketch a graph for this function. \begin{align*}x\end{align*} 
\begin{align*}y\end{align*} 
4  
2  
0  
2  
4 
5. Is the function you graphed in the previous question
a linear function or a nonlinear function?6. The equation
is a function. Complete the table below to identify five ordered pairs for this function. Plot those points on the coordinate plane below. Then connect those points to sketch a graph for this function.\begin{align*}x\end{align*}  \begin{align*}y\end{align*} 
2  
1  
0  
1  
2 
7. Is the function you graphed in the previous question
a linear function or a nonlinear function?The rule for a linear function is: add 1 to each
value to find each value.8. Write an equation that represents this linear function.
9. Graph the function on this coordinate plane.
The rule for a linear function is: multiply each
value by 2 and then subtract 2 to find each value.10. Write an equation that represents this linear function.
11. Graph the function on this coordinate plane.