Which Line is Parallel to the Line 8x 2y 12
Graphs
36
Use the Slope–Intercept Form of an Equation of a Line
Learning Objectives
By the end of this section, you will be able to:
 Recognize the relation between the graph and the slope–intercept form of an equation of a line
 Identify the slope and yintercept form of an equation of a line
 Graph a line using its slope and intercept
 Choose the most convenient method to graph a line
 Graph and interpret applications of slope–intercept
 Use slopes to identify parallel lines
 Use slopes to identify perpendicular lines
Recognize the Relation Between the Graph and the Slope–Intercept Form of an Equation of a Line
We have graphed linear equations by plotting points, using intercepts, recognizing horizontal and vertical lines, and using the point–slope method. Once we see how an equation in slope–intercept form and its graph are related, we’ll have one more method we can use to graph lines.
In Graph Linear Equations in Two Variables, we graphed the line of the equation
by plotting points. See (Figure). Let’s find the slope of this line.
The red lines show us the rise is 1 and the run is 2. Substituting into the slope formula:
What is the
yintercept of the line? The
yintercept is where the line crosses the
yaxis, so
yintercept is
. The equation of this line is:
Notice, the line has:
When a linear equation is solved for
, the coefficient of the
term is the slope and the constant term is the
ycoordinate of the
yintercept. We say that the equation
is in slope–intercept form.
SlopeIntercept Form of an Equation of a Line
The
slope–intercept form
of an equation of a line with slope
and
yintercept,
is,
Sometimes the slope–intercept form is called the “yform.”
Use the graph to find the slope and
yintercept of the line,
.
Compare these values to the equation.
Use the graph to find the slope and
yintercept of the line
. Compare these values to the equation
.
slope
and
yintercept
Use the graph to find the slope and
yintercept of the line
. Compare these values to the equation
.
slope
and
yintercept
Identify the Slope and
yIntercept From an Equation of a Line
In Understand Slope of a Line, we graphed a line using the slope and a point. When we are given an equation in slope–intercept form, we can use the
yintercept as the point, and then count out the slope from there. Let’s practice finding the values of the slope and
yintercept from the equation of a line.
Identify the slope and
yintercept of the line with equation
.
Solution
We compare our equation to the slope–intercept form of the equation.


Write the equation of the line. 

Identify the slope. 

Identify the yintercept. 

Identify the slope and
yintercept of the line
.
Identify the slope and
yintercept of the line
.
When an equation of a line is titinada given in slope–intercept form, our first step will be to solve the equation for
.
Identify the slope and
yintercept of the line with equation
.
Identify the slope and
yintercept of the line
.
Identify the slope and
yintercept of the line
.
Graph a Line Using its Slope and Intercept
Now that we know how to find the slope and
yintercept of a line from its equation, we can graph the line by plotting the
yintercept and then using the slope to find another point.
How to Graph a Line Using its Slope and Intercept
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph a line using its slope and
yintercept.
 Find the slopeintercept form of the equation of the line.
 Identify the slope and
yintercept.  Plot the
yintercept.  Use the slope formula
to identify the rise and the run.  Starting at the
yintercept, count out the rise and run to mark the second point.  Connect the points with a line.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
We have used a grid with
and
both going from about
to 10 for all the equations we’ve graphed so far. Not all linear equations can be graphed on this small grid. Often, especially in applications with realworld data, we’ll need to extend the axes to bigger positive or smaller negative numbers.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Graph the line of the equation
using its slope and
yintercept.
Now that we have graphed lines by using the slope and
yintercept, let’s summarize all the methods we have used to graph lines. See (Figure).
Choose the Most Convenient Method to Graph a Line
Now that we have seen several methods we can use to graph lines, how do we know which method to use for a given equation?
While we could plot points, use the slope–intercept form, or find the intercepts for
any
equation, if we recognize the most convenient way to graph a certain type of equation, our work will be easier. Generally, plotting points is not the most efficient way to graph a line. We saw better methods in sections 4.3, 4.4, and earlier in this section. Let’s look for some patterns to help determine the most convenient method to graph a line.
Here are six equations we graphed in this chapter, and the method we used to graph each of them.
Equations #1 and #2 each have just one variable. Remember, in equations of this form the value of that one variable is constant; it does not depend on the value of the other variable. Equations of this form have graphs that are vertical or mengufuk lines.
In equations #3 and #4, both
and
are on the same side of the equation. These two equations are of the form
. We substituted
to find the
xintercept and
to find the
yintercept, and then found a third point by choosing another value for
or
.
Equations #5 and #6 are written in slope–intercept form. After identifying the slope and
yintercept from the equation we used them to graph the line.
This leads to the following strategy.
Strategy for Choosing the Most Convenient Method to Graph a Line
Consider the form of the equation.
Determine the most convenient method to graph each line.
ⓐ
ⓑ
ⓒ
ⓓ
.
Determine the most convenient method to graph each line:
ⓐ
ⓑ
ⓒ
ⓓ
.
ⓐ
interceptsⓑ
mengufuk lineⓒ
slope–interceptⓓ
vertical line
Determine the most convenient method to graph each line:
ⓐ
ⓑ
ⓒ
ⓓ
.
ⓐ
vertical lineⓑ
slope–interceptⓒ
horizontal lineⓓ
intercepts
Graph and Interpret Applications of Slope–Intercept
Many realworld applications are modeled by linear equations. We will take a look at a few applications here so you can see how equations written in slope–intercept form relate to realworld situations.
Usually when a linear equation models a realworld situation, different letters are used for the variables, instead of
x
and
y. The variable names remind us of what quantities are being measured.
The equation
is used to convert temperatures,
, on the Celsius scale to temperatures,
, on the Fahrenheit scale.
ⓐ
Find the Fahrenheit temperature for a Celsius temperature of 0.
ⓑ
Find the Fahrenheit temperature for a Celsius temperature of 20.
ⓒ
Interpret the slope and
Fintercept of the equation.
ⓓ
Graph the equation.
Solution
ⓐ Find the Fahrenheit temperature for a Celsius temperature of 0. Find when . Simplify. 

ⓑ Find the Fahrenheit temperature for a Celsius temperature of 20. Find when . Simplify. Simplify. 
ⓒ
Interpret the slope and
Fintercept of the equation.
Even though this equation uses
and
, it is still in slope–intercept form.
The slope,
, means that the temperature Fahrenheit (F) increases 9 degrees when the temperature Celsius (C) increases 5 degrees.
The
Fintercept means that when the temperature is
on the Celsius scale, it is
on the Fahrenheit scale.
ⓓ
Graph the equation.
We’ll need to use a larger scale than our usual. Tiba at the
Fintercept
then count out the rise of 9 and the run of 5 to get a second point. See (Figure).
The equation
is used to estimate a woman’s height in inches,
h, based on her shoe size,
s.
ⓐ
Estimate the height of a child who wears women’s shoe size 0.
ⓑ
Estimate the height of a woman with shoe size 8.
ⓒ
Interpret the slope and
hintercept of the equation.
ⓓ
Graph the equation.

ⓐ
50 inches 
ⓑ
66 inches 
ⓒ
The slope, 2, means that the height,
h, increases by 2 inches when the shoe size,
s, increases by 1. The
hintercept means that when the shoe size is 0, the height is 50 inches. 
ⓓ
The equation
is used to estimate the temperature in degrees Fahrenheit,
Cakrawala, based on the number of cricket chirps,
n, in one minute.
ⓐ
Estimate the temperature when there are no chirps.
ⓑ
Estimate the temperature when the number of chirps in one minute is 100.
ⓒ
Interpret the slope and
Horizonintercept of the equation.
ⓓ
Graph the equation.
The cost of running some types business has two components—a
fixed cost
and a
variable cost. The fixed cost is always the same regardless of how many units are produced. This is the cost of rent, insurance, equipment, advertising, and other items that must be paid regularly. The variable cost depends on the number of units produced. It is for the material and labor needed to produce each item.
Stella has a home business selling gourmet pizzas. The equation
models the relation between her weekly cost,
C, in dollars and the number of pizzas,
p, that she sells.
ⓐ
Find Stella’s cost for a week when she sells no pizzas.
ⓑ
Find the cost for a week when she sells 15 pizzas.
ⓒ
Interpret the slope and
Cintercept of the equation.
ⓓ
Graph the equation.
Sam drives a delivery van. The equation
models the relation between his weekly cost,
C, in dollars and the number of miles,
m, that he drives.
ⓐ
Find Sam’s cost for a week when he drives 0 miles.
ⓑ
Find the cost for a week when he drives 250 miles.
ⓒ
Interpret the slope and
Cintercept of the equation.
ⓓ
Graph the equation.

ⓐ
?60 
ⓑ
?185 
ⓒ
The slope, 0.5, means that the weekly cost,
C, increases by ?0.50 when the number of miles driven,
horizon,
increases by 1. The
Cintercept means that when the number of miles driven is 0, the weekly cost is ?60 
ⓓ
Loreen has a calligraphy business. The equation
models the relation between her weekly cost,
C, in dollars and the number of wedding invitations,
n, that she writes.
ⓐ
Find Loreen’s cost for a week when she writes no invitations.
ⓑ
Find the cost for a week when she writes 75 invitations.
ⓒ
Interpret the slope and
Cintercept of the equation.
ⓓ
Graph the equation.

ⓐ
?35 
ⓑ
?170 
ⓒ
The slope, 1.8, means that the weekly cost, C, increases by ?1.80 when the number of invitations,
n, increases by 1.80.
The
Cintercept means that when the number of invitations is 0, the weekly cost is ?35.; 
ⓓ
Use Slopes to Identify Parallel Lines
The slope of a line indicates how steep the line is and whether it rises or falls as we read it from left to right. Two lines that have the same slope are called parallel lines. Parallel lines never intersect.
We say this more formally in terms of the rectangular coordinate system. Two lines that have the same slope and different
yintercepts are called
parallel lines. See (Figure).
What about vertical lines? The slope of a vertical line is undefined, so vertical lines don’kaki langit fit in the definition above. We say that vertical lines that have different
xintercepts are parallel. See (Figure).
Parallel Lines
Parallel lines are lines in the same plane that do not intersect.
Let’s graph the equations
and
on the same grid. The first equation is already in slope–intercept form:
. We solve the second equation for
:
Graph the lines.
Notice the lines look parallel. What is the slope of each line? What is the
yintercept of each line?
The slopes of the lines are the same and the
yintercept of each line is different. So we know these lines are parallel.
Since parallel lines have the same slope and different
yintercepts, we can now just look at the slope–intercept form of the equations of lines and decide if the lines are parallel.
Use slopes and
yintercepts to determine if the lines
and
are parallel.
Use slopes and
yintercepts to determine if the lines
are parallel.
parallel
Use slopes and
yintercepts to determine if the lines
are parallel.
parallel
Use slopes and
yintercepts to determine if the lines
and
are parallel.
Use slopes and
yintercepts to determine if the lines
are parallel.
parallel
Use slopes and
yintercepts to determine if the lines
are parallel.
parallel
Use slopes and
yintercepts to determine if the lines
and
are parallel.
Solution
Since there is no, the equations cannot be put in slope–intercept form. But we recognize them as equations of vertical lines. Their
xintercepts are
and
. Since their
xintercepts are different, the vertical lines are parallel.
Use slopes and
yintercepts to determine if the lines
and
are parallel.
parallel
Use slopes and
yintercepts to determine if the lines
and
are parallel.
parallel
Use slopes and
yintercepts to determine if the lines
and
are parallel. You may want to graph these lines, too, to see what they look like.
Use slopes and
yintercepts to determine if the lines
and
are parallel.
not parallel; same line
Use slopes and
yintercepts to determine if the lines
and
are parallel.
not parallel; same line
Use Slopes to Identify Perpendicular Lines
Let’s look at the lines whose equations are
and
, shown in (Figure).
These lines lie in the same plane and intersect in right angles. We call these lines
perpendicular.
What do you notice about the slopes of these two lines? As we read from left to right, the line
rises, so its slope is positive. The line
drops from left to right, so it has a negative slope. Does it make sense to you that the slopes of two perpendicular lines will have opposite signs?
If we look at the slope of the first line,
, and the slope of the second line,
, we can see that they are
negative reciprocals
of each other. If we multiply them, their product is
This is always true for
perpendicular lines
and leads us to this definition.
Perpendicular Lines
Perpendicular lines
are lines in the same plane that form a right angle.
If
are the slopes of two perpendicular lines, then:
Vertical lines and horizontal lines are always perpendicular to each other.
We were able to look at the slope–intercept form of linear equations and determine whether or not the lines were parallel. We can do the same thing for perpendicular lines.
We find the slope–intercept form of the equation, and then see if the slopes are negative reciprocals. If the product of the slopes is
, the lines are perpendicular. Perpendicular lines may have the same
yintercepts.
Use slopes to determine if the lines,
and
are perpendicular.
Solution
The first equation is already in slopeintercept form.  
Solve the second equation for . 

Identify the slope of each line. 
The slopes are negative reciprocals of each other, so the lines are perpendicular. We check by multiplying the slopes,
Use slopes to determine if the lines
and
are perpendicular.
perpendicular
Use slopes to determine if the lines
and
are perpendicular.
perpendicular
Use slopes to determine if the lines,
and
are perpendicular.
Use slopes to determine if the lines
and
are perpendicular.
not perpendicular
Use slopes to determine if the lines
and
are perpendicular.
titinada perpendicular
Key Concepts
Practice Makes Perfect
Recognize the Relation Between the Graph and the Slope–Intercept Form of an Equation of a Line
In the following exercises, use the graph to find the slope and yintercept of each line. Compare the values to the equation
.
Identify the Slope and yIntercept From an Equation of a Line
In the following exercises, identify the slope and yintercept of each line.
Graph a Line Using Its Slope and Intercept
In the following exercises, graph the line of each equation using its slope and yintercept.
Choose the Most Convenient Method to Graph a Line
In the following exercises, determine the most convenient method to graph each line.
horizontal line
vertical line
slope–intercept
intercepts
slope–intercept
horizontal line
intercepts
Graph and Interpret Applications of Slope–Intercept
The equation
models the relation between the amount of Tuyet’s monthly water bill payment,
P, in dollars, and the number of units of water,
w, used.

ⓐ
Find Tuyet’s payment for a month when 0 units of water are used. 
ⓑ
Find Tuyet’s payment for a month when 12 units of water are used. 
ⓒ
Interpret the slope and
Pintercept of the equation. 
ⓓ
Graph the equation.
The equation
models the relation between the amount of Randy’s monthly water bill payment,
P, in dollars, and the number of units of water,
w, used.

ⓐ
Find the payment for a month when Randy used 0 units of water. 
ⓑ
Find the payment for a month when Randy used 15 units of water. 
ⓒ
Interpret the slope and
Pintercept of the equation. 
ⓓ
Graph the equation.

ⓐ
?28 
ⓑ
?66.10 
ⓒ
The slope, 2.54, means that Randy’s payment,
P, increases by ?2.54 when the number of units of water he used,
w,
increases by 1. The
P–intercept means that if the number units of water Randy used was 0, the payment would be ?28. 
ⓓ
Bruce drives his car for his job. The equation
models the relation between the amount in dollars,
R, that he is reimbursed and the number of miles,
m, he drives in one day.

ⓐ
Find the amount Bruce is reimbursed on a day when he drives 0 miles. 
ⓑ
Find the amount Bruce is reimbursed on a day when he drives 220 miles. 
ⓒ
Interpret the slope and
Rintercept of the equation. 
ⓓ
Graph the equation.
Janelle is planning to rent a car while on vacation. The equation
models the relation between the cost in dollars,
C, sendirisendiri day and the number of miles,
m, she drives in one day.

ⓐ
Find the cost if Janelle drives the car 0 miles one day. 
ⓑ
Find the cost on a day when Janelle drives the car 400 miles. 
ⓒ
Interpret the slope and
C–intercept of the equation. 
ⓓ
Graph the equation.

ⓐ
?15 
ⓑ
?143 
ⓒ
The slope, 0.32, means that the cost,
C, increases by ?0.32 when the number of miles driven,
m,
increases by 1. The
Cintercept means that if Janelle drives 0 miles one day, the cost would be ?15. 
ⓓ
Cherie works in retail and her weekly salary includes commission for the amount she sells. The equation
models the relation between her weekly salary,
S, in dollars and the amount of her sales,
c, in dollars.

ⓐ
Find Cherie’s salary for a week when her sales were 0. 
ⓑ
Find Cherie’s salary for a week when her sales were 3600. 
ⓒ
Interpret the slope and
S–intercept of the equation. 
ⓓ
Graph the equation.
Patel’s weekly salary includes a base pay plus commission on his sales. The equation
models the relation between his weekly salary,
S, in dollars and the amount of his sales,
c, in dollars.

ⓐ
Find Patel’s salary for a week when his sales were 0. 
ⓑ
Find Patel’s salary for a week when his sales were 18,540. 
ⓒ
Interpret the slope and
Sintercept of the equation. 
ⓓ
Graph the equation.

ⓐ
?750 
ⓑ
?2418.60 
ⓒ
The slope, 0.09, means that Patel’s salary,
S, increases by ?0.09 for every ?1 increase in his sales. The
Sintercept means that when his sales are ?0, his salary is ?750. 
ⓓ
Costa is planning a lunch banquet. The equation
models the relation between the cost in dollars,
C, of the banquet and the number of guests,
g.

ⓐ
Find the cost if the number of guests is 40. 
ⓑ
Find the cost if the number of guests is 80. 
ⓒ
Interpret the slope and
Cintercept of the equation. 
ⓓ
Graph the equation.
Margie is planning a dinner banquet. The equation
models the relation between the cost in dollars,
C
of the banquet and the number of guests,
g.

ⓐ
Find the cost if the number of guests is 50. 
ⓑ
Find the cost if the number of guests is 100. 
ⓒ
Interpret the slope and
C–intercept of the equation. 
ⓓ
Graph the equation.

ⓐ
?2850 
ⓑ
?4950 
ⓒ
The slope, 42, means that the cost,
C, increases by ?42 for when the number of guests increases by 1. The
Cintercept means that when the number of guests is 0, the cost would be ?750. 
ⓓ
Use Slopes to Identify Parallel Lines
In the following exercises, use slopes and yintercepts to determine if the lines are parallel.
parallel
parallel
not parallel
parallel
parallel
parallel
parallel
parallel
not parallel
not parallel
not parallel
not parallel
not parallel
Use Slopes to Identify Perpendicular Lines
In the following exercises, use slopes and yintercepts to determine if the lines are perpendicular.
perpendicular
perpendicular
titinada perpendicular
not perpendicular
perpendicular
perpendicular
Everyday Math
The equation
can be used to convert temperatures
F, on the Fahrenheit scale to temperatures,
C, on the Celsius scale.

ⓐ
Explain what the slope of the equation means. 
ⓑ
Explain what the
C–intercept of the equation means.
The equation
is used to estimate the number of cricket chirps,
n, in one minute based on the temperature in degrees Fahrenheit,
T.

ⓐ
Explain what the slope of the equation means. 
ⓑ
Explain what the
n–intercept of the equation means. Is this a realistic situation?
Writing Exercises
Explain in your own words how to decide which method to use to graph a line.
Why are all melintang lines parallel?
Answers will vary.
Self Check
ⓐ
After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.
ⓑ
After looking at the checklist, do you think you are wellprepared for the next section? Why or why titinada?
Which Line is Parallel to the Line 8x 2y 12
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