The cost per month for a cell-phone plan is $60 plus $7.50 for every gigabyte (GB) of data you use. (For billing purposes, actual usage is rounded to the nearest one-quarter GB.) Write an equation for the cost of the data plan and determine how much your bill will be if you use 4.5 GB of data in a month.

### Guidance

From the previous lesson, we know that the equation of a line is , where is the slope and is the intercept. From these two pieces of information we can graph any line.

#### Example A

Graph on the Cartesian plane.

**
Solution:
**
First, the Cartesian plane is the
plane. Typically, when graphing lines, draw each axis from -10 to 10. To graph this line, you need to find the slope and
intercept. By looking at the equation,
is the slope and 4, or (0, 4), is the
intercept. To start graphing this line, plot the
intercept on the
axis.

Now, we need to use the slope to find the next point on the line. Recall that the slope is also , so for , we will rise 1 and run 3 from the intercept. Do this a couple of times to get at least three points.

Now that we have three points, connect them to form the line .

#### Example B

Graph .

**
Solution:
**
Now that the slope is negative, the vertical distance will “fall” instead of rise. Also, because the slope is a whole number, we need to put it over 1. Therefore, for a slope of -4, the line will fall 4 and run 1 OR rise 4 and run backward 1. Start at the
intercept, and then use the slope to find a few more points.

#### Example C

Graph .

**
Solution:
**
Any line in the form
is a vertical line. To graph any vertical line, plot the value, in this case 5, on the
axis. Then draw the vertical line.

To graph a horizontal line, , it will be the same process, but plot the value given on the axis and draw a horizontal line.

**
Intro Problem Revisit
**
If
*
x
*
is the number of GB of data you use in a month and
*
y
*
is the total cost you pay, then the equation for the cell-phone plan would be
. If you use 4.5 GB in a month, the total cost would be
.

So your bill for the month would be $93.75.

### Guided Practice

Graph the following lines.

1.

2.

3.

#### Answers

All the answers are on the same grid below.

1. Plot (0, 2) and the slope is -1, which means you fall 1 and run 1.

2. Plot (0, -1) and then rise 3 and run 4 to the next point, (4, 2).

3. Plot -6 on the axis and draw a horizontal line.

### Explore More

Graph the following lines in the Cartesian plane.

- Graph and on the same set of axes. Where do they intersect?
- If you were to make a general rule for the lines and , where will they always intersect?
- The cost per month, (in dollars), of placing an ad on a website is , where is the number of times someone clicks on your link. How much would it cost you if 500 people clicked on your link?