# Parallel And Perpendicular Lines Worksheet Page 24

WRITE EQUATIONS TO SOLVE PROBLEMS
Many real-world situations
can be modeled using linear equations. In many business applications, the slope
represents a rate.
Write Linear Equations
Example
Example
5
5
CELL PHONE COSTS
Martina’s current cellular phone plan charges \$14.95
per month and \$0.10 per minute of air time.
a. Write an equation to represent the total monthly cost C for t minutes of
air time.
For each minute of air time, the cost increases \$0.10. So, the rate of change, or
slope, is 0.10. The y-intercept is located where 0 minutes of air time are used,
or \$14.95.
C
mt
b
Slope-intercept form
C
0.10t
14.95
m
0.10, b
14.95
The total monthly cost can be represented by the equation C
0.10t
14.95.
b. Compare her current plan to the plan presented at the beginning of the
lesson. If she uses an average of 40 minutes of air time each month, which
plan offers the better rate?
Evaluate each equation for t
40.
Current plan:
C
0.10t
14.95
0.10(40)
14.95
t
40
18.95
Simplify.
Alternate plan: C
0.07t
19.95
0.07(40)
19.95
t
40
22.75
Simplify.
Given her average usage, Martina’s current plan offers the better rate.
2
Concept Check
1. Explain how you would write an equation of a line whose slope is
that
5
contains ( 2, 8).
2. Write equations in slope-intercept form for two lines that contain ( 1,
5).
OPEN ENDED
3.
Graph a line that is not horizontal or vertical on the coordinate
plane. Write the equation of the line.
Guided Practice
Write an equation in slope-intercept form of the line having the given slope and
y-intercept.
GUIDED PRACTICE KEY
1
3
4. m
5. m
6. m
3
2
5
y-intercept: 4
intercept at (0,
2)
y-intercept:
4
Write an equation in point-slope form of the line having the given slope that
contains the given point.
3
7. m
, (4,
1)
8. m
3, (7, 5)
9. m
1.25, (20, 137.5)
2
Lesson 3-4 Equations of Lines 147