The cost per month for a cellphone plan is $60 plus $7.50 for every gigabyte (GB) of data you use. (For billing purposes, actual usage is rounded to the nearest onequarter 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
Example A
Graph
Solution: First, the Cartesian plane is the
Now, we need to use the slope to find the next point on the line. Recall that the slope is also
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
Example C
Graph
Solution: Any line in the form
To graph 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 cellphone plan 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
Explore More
Graph the following lines in the Cartesian plane.

y=−2x−3 
y=x+4 
y=13x−1 
y=9 
y=−25x+7 
y=24x−5 
y=−5x−2 
y=−x 
y=4 
x=−3 
y=32x+3 
y=−16x−8  Graph
y=4 andx=−6 on the same set of axes. Where do they intersect?  If you were to make a general rule for the lines
y=b andx=a , where will they always intersect?  The cost per month,
C (in dollars), of placing an ad on a website isC=0.25x+50 , wherex is the number of times someone clicks on your link. How much would it cost you if 500 people clicked on your link?