3. Equations and their Graphs
3.5 Use the Slope–Intercept Form of an Equation of a Line -optional
Learning Objectives
By the end of this section it is expected that 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 y-intercept 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.
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 y-intercept of the line? The y-intercept is where the line crosses the y-axis, so y-intercept 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 y-coordinate of the y-intercept. We say that the equation is in slope–intercept form.
Slope-intercept form of an equation of a line
The slope–intercept form of an equation of a line with slope and y-intercept, is,
Sometimes the slope–intercept form is called the “y-form.”
EXAMPLE 1
Use the graph to find the slope and y-intercept of the line, .
Compare these values to the equation.
To find the slope of the line, we need to choose two points on the line. We’ll use the points and .
| Find the rise and run. | |
| Find the y-intercept of the line. | The y-intercept is the point (0, 1). |
The slope is the same as the coefficient of and the y-coordinate of the y-intercept is the same as the constant term.
TRY IT 1
Use the graph to find the slope and y-intercept of the line . Compare these values to the equation .
Show answer
slope and y-intercept
Identify the Slope and y-Intercept From an Equation of a Line
In the last sub-chapter, we graphed a line using the slope and a point. When we are given an equation in slope–intercept form, we can use the y-intercept as the point, and then count out the slope from there. Let’s practice finding the values of the slope and y-intercept from the equation of a line.
EXAMPLE 2
Identify the slope and y-intercept of the line with equation .
We compare our equation to the slope–intercept form of the equation.
| Write the equation of the line. | |
| Identify the slope. | |
| Identify the y-intercept. |
TRY IT 2
Identify the slope and y-intercept of the line .
Show answer
When an equation of a line is not given in slope–intercept form, our first step will be to solve the equation for .
EXAMPLE 3
Identify the slope and y-intercept of the line with equation .
This equation is not in slope–intercept form. In order to compare it to the slope–intercept form we must first solve the equation for.
| Solve for y. | |
| Subtract x from each side. | |
| Divide both sides by 2. | |
| Simplify. | |
| (Remember:) | |
| Simplify. | |
| Write the slope–intercept form of the equation of the line. | |
| Write the equation of the line. | |
| Identify the slope. | |
| Identify the y-intercept. |
TRY IT 3
Identify the slope and y-intercept of the line .
Show answer
Graph a Line Using its Slope and Intercept
Now that we know how to find the slope and y-intercept of a line from its equation, we can graph the line by plotting the y-intercept and then using the slope to find another point.
EXAMPLE 4
Graph the line of the equation using its slope and y-intercept.
TRY IT 4
Graph the line of the equation using its slope and y-intercept.
Show answer
HOW TO: Graph a line using its slope and y-intercept
- Find the slope-intercept form of the equation of the line.
- Identify the slope and y-intercept.
- Plot the y-intercept.
- Use the slope formula to identify the rise and the run.
- Starting at the y-intercept, count out the rise and run to mark the second point.
- Connect the points with a line.
EXAMPLE 5
Graph the line of the equation using its slope and y-intercept.
| The equation is in slope–intercept form. | |
| Identify the slope and y-intercept. | |
| y-intercept is (0, 4) | |
| Plot the y-intercept. | See graph below. |
| Identify the rise and the run. | |
| Count out the rise and run to mark the second point. | rise −1, run 1 |
| Draw the line. | |
| To check your work, you can find another point on the line and make sure it is a solution of the equation. In the graph we see the line goes through (4, 0). | |
| Check. |
TRY IT 5
Graph the line of the equation using its slope and y-intercept.
Show answer
EXAMPLE 6
Graph the line of the equation using its slope and y-intercept.
Solution
| The equation is in slope–intercept form. | |
| Identify the slope and y-intercept. | ; y-intercept is (0, −3) |
| Plot the y-intercept. | See graph below. |
| Identify the rise and the run. | |
| Count out the rise and run to mark the second point. | |
| Draw the line. |
TRY IT 6
Graph the line of the equation using its slope and y-intercept.
Show answer
EXAMPLE 7
Graph the line of the equation using its slope and y-intercept.
| Find the slope–intercept form of the equation. | |
| The equation is now in slope–intercept form. | |
| Identify the slope and y-intercept. | |
| y-intercept is (0, −4) | |
| Plot the y-intercept. | See graph below. |
| Identify the rise and the run; count out the rise and run to mark the second point. | |
| Draw the line. |
TRY IT 7
Graph the line of the equation using its slope and y-intercept.
Show answer
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 real-world data, we’ll need to extend the axes to bigger positive or smaller negative numbers.
EXAMPLE 8
Graph the line of the equation using its slope and y-intercept.
Solution
We’ll use a grid with the axes going from about to 80.
| The equation is in slope–intercept form. | |
| Identify the slope and y-intercept. | |
| The y-intercept is (0, 45) | |
| Plot the y-intercept. | See graph below. |
| Count out the rise and run to mark the second point. The slope is ; in fraction form this means . Given the scale of our graph, it would be easier to use the equivalent fraction . | |
| Draw the line. |
TRY IT 8
Graph the line of the equation using its slope and y-intercept.
Show answer
Methods to graph lines
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 horizontal 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 x-intercept and to find the y-intercept, 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 y-intercept 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.
- If it only has one variable, it is a vertical or horizontal line.
- is a vertical line passing through the x-axis at .
- is a horizontal line passing through the y-axis at .
- If is isolated on one side of the equation, in the form , graph by using the slope and y-intercept.
- Identify the slope and y-intercept and then graph.
- If the equation is of the form , find the intercepts.
- Find the x– and y-intercepts, a third point, and then graph.
EXAMPLE 9
Determine the most convenient method to graph each line.
a) b) c) d) .
a)
This equation has only one variable,. Its graph is a horizontal line crossing the y-axis at .
b)
This equation is of the form . The easiest way to graph it will be to find the intercepts and one more point.
c)
There is only one variable, . The graph is a vertical line crossing the x-axis at 7.
d)
Since this equation is in form, it will be easiest to graph this line by using the slope and y-intercept.
TRY IT 9
Determine the most convenient method to graph each line: a) b) c) d) .
Show answer
a) intercepts b) horizontal line c) slope–intercept d) vertical line
Graph and Interpret Applications of Slope–Intercept
Many real-world 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 real-world situations.
Usually when a linear equation models a real-world situation, different letters are used for the variables, instead of x and y. The variable names remind us of what quantities are being measured.
EXAMPLE 10
The equation is used to convert temperatures, , on the Celsius scale to temperatures, , on the Fahrenheit scale.
a) Find the Fahrenheit temperature for a Celsius temperature of 0.
b) Find the Fahrenheit temperature for a Celsius temperature of 20.
c) Interpret the slope and F-intercept of the equation.
d) Graph the equation.
| a) Find the Fahrenheit temperature for a Celsius temperature of 0. Find when . Simplify. |
|
| b) Find the Fahrenheit temperature for a Celsius temperature of 20. Find when . Simplify. Simplify. |
c) Interpret the slope and F-intercept 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 F-intercept means that when the temperature is 0° on the Celsius scale, it is 32° on the Fahrenheit scale.
d) Graph the equation.
We’ll need to use a larger scale than our usual. Start at the F-intercept then count out the rise of 9 and the run of 5 to get a second point. See (Figure).
TRY IT 10
The equation is used to estimate a woman’s height in inches, h, based on her shoe size, s.
a) Estimate the height of a child who wears women’s shoe size 0.
b) Estimate the height of a woman with shoe size 8.
c) Interpret the slope and h-intercept of the equation.
d) Graph the equation.
Show answer
a) 50 inches
b) 66 inches
c) The slope, 2, means that the height, h, increases by 2 inches when the shoe size, s, increases by 1. The h-intercept means that when the shoe size is 0, the height is 50 inches.
d)
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.
EXAMPLE 11
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.
a) Find Stella’s cost for a week when she sells no pizzas.
b) Find the cost for a week when she sells 15 pizzas.
c) Interpret the slope and C-intercept of the equation.
d) Graph the equation.
| a) Find Stella’s cost for a week when she sells no pizzas. | |
| Find C when . | |
| Simplify. | |
| Stella’s fixed cost is $25 when she sells no pizzas. | |
| b) Find the cost for a week when she sells 15 pizzas. | |
| Find C when . | |
| Simplify. | |
| Stella’s costs are $85 when she sells 15 pizzas. | |
| c) Interpret the slope and C-intercept of the equation. | |
| The slope, 4, means that the cost increases by $4 for each pizza Stella sells. The C-intercept means that even when Stella sells no pizzas, her costs for the week are $25. | |
| d) Graph the equation. We’ll need to use a larger scale than our usual. Start at the C-intercept (0, 25) then count out the rise of 4 and the run of 1 to get a second point. |
TRY IT 11
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 C-intercept of the equation.
ⓓ Graph the equation.
Show answer
- ⓐ $60
- ⓑ $185
- ⓒ The slope, 0.5, means that the weekly cost, C, increases by $0.50 when the number of miles driven, n, increases by 1. The C-intercept means that when the number of miles driven is 0, the weekly cost is $60
- ⓓ
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 y-intercepts are called parallel lines. See (Figure).
Verify that both lines have the same slope, , and different y-intercepts.
What about vertical lines? The slope of a vertical line is undefined, so vertical lines don’t fit in the definition above. We say that vertical lines that have different x-intercepts are parallel. See (Figure).
Vertical lines with different x-intercepts are parallel.
Parallel lines
Parallel lines are lines in the same plane that do not intersect.
- Parallel lines have the same slope and different y-intercepts.
- If and are the slopes of two parallel lines then.
- Parallel vertical lines have different x-intercepts.
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 y-intercept of each line?
The slopes of the lines are the same and the y-intercept of each line is different. So we know these lines are parallel.
Since parallel lines have the same slope and different y-intercepts, we can now just look at the slope–intercept form of the equations of lines and decide if the lines are parallel.
EXAMPLE 12
Use slopes and y-intercepts to determine if the lines and are parallel.
| Solve the first equation for . | and | ||
| The equation is now in slope-intercept form. | |||
| The equation of the second line is already in slope-intercept form. | |||
| Identify the slope and -intercept of both lines. | |||
| y-intercept is (0, −3) | y-intercept is (0, 1) |
The lines have the same slope and different y-intercepts and so they are parallel. You may want to graph the lines to confirm whether they are parallel.
TRY IT 12
Use slopes and y-intercepts to determine if the lines and are parallel.
Show answer
parallel
EXAMPLE 13
Use slopes and y-intercepts to determine if the lines and are parallel.
| and | |||
| Write each equation in slope-intercept form. | |||
| Since there is no term we write . | |||
| Identify the slope and -intercept of both lines. | |||
| y-intercept is (0, 4) | y-intercept is (0, 3) |
The lines have the same slope and different y-intercepts and so they are parallel.
There is another way you can look at this example. If you recognize right away from the equations that these are horizontal lines, you know their slopes are both 0. Since the horizontal lines cross the y-axis at and at , we know the y-intercepts are and . The lines have the same slope and different y-intercepts and so they are parallel.
TRY IT 13
Use slopes and y-intercepts to determine if the lines and are parallel.
Show answer
parallel
EXAMPLE 14
Use slopes and y-intercepts to determine if the lines and are parallel.
Since there is no, the equations cannot be put in slope–intercept form. But we recognize them as equations of vertical lines. Their x-intercepts are and . Since their x-intercepts are different, the vertical lines are parallel.
TRY IT 14
Use slopes and y-intercepts to determine if the lines and are parallel.
Show answer
parallel
EXAMPLE 15
Use slopes and y-intercepts to determine if the lines and are parallel. You may want to graph these lines, too, to see what they look like.
| and | |||
| The first equation is already in slope-intercept form. | |||
| Solve the second equation for . | |||
| The second equation is now in slope-intercept form. | |||
| Identify the slope and -intercept of both lines. | |||
The lines have the same slope, but they also have the same y-intercepts. Their equations represent the same line. They are not parallel; they are the same line.
TRY IT 15
Use slopes and y-intercepts to determine if the lines and are parallel.
Show answer
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 and are the slopes of two perpendicular lines, then:
and
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 y-intercepts.
EXAMPLE 16
Use slopes to determine if the lines, and are perpendicular.
| The first equation is already in slope-intercept 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,
TRY IT 16
Use slopes to determine if the lines and are perpendicular.
Show answer
perpendicular
EXAMPLE 17
Use slopes to determine if the lines, and are perpendicular.
| Solve the equations for . | ||
| Identify the slope of each line. |
The slopes are reciprocals of each other, but they have the same sign. Since they are not negative reciprocals, the lines are not perpendicular.
TRY IT 17
Use slopes to determine if the lines and are perpendicular.
Show answer
not perpendicular
Access this online resource for additional instruction and practice with graphs.
Key Concepts
- The slope–intercept form of an equation of a line with slope and y-intercept, is, .
- Graph a Line Using its Slope and y-Intercept
- Find the slope-intercept form of the equation of the line.
- Identify the slope and y-intercept.
- Plot the y-intercept.
- Use the slope formula to identify the rise and the run.
- Starting at the y-intercept, count out the rise and run to mark the second point.
- Connect the points with a line.
- Strategy for Choosing the Most Convenient Method to Graph a Line: Consider the form of the equation.
- If it only has one variable, it is a vertical or horizontal line.
is a vertical line passing through the x-axis at .
is a horizontal line passing through the y-axis at . - If is isolated on one side of the equation, in the form , graph by using the slope and y-intercept.
Identify the slope and y-intercept and then graph. - If the equation is of the form , find the intercepts.
Find the x– and y-intercepts, a third point, and then graph.
- If it only has one variable, it is a vertical or horizontal line.
- Parallel lines are lines in the same plane that do not intersect.
- Parallel lines have the same slope and different y-intercepts.
- If m1 and m2 are the slopes of two parallel lines then
- Parallel vertical lines have different x-intercepts.
- Perpendicular lines are lines in the same plane that form a right angle.
- If are the slopes of two perpendicular lines, then and .
- Vertical lines and horizontal lines are always perpendicular to each other.
Glossary
- parallel lines
- Lines in the same plane that do not intersect.
- perpendicular lines
- Lines in the same plane that form a right angle.
- slope-intercept form of an equation of a line
- The slope–intercept form of an equation of a line with slope and y-intercept, is, .
3.5 Exercise Set
In the following exercises, use the graph to find the slope and y-intercept of each line. Compare the values to the equation .
| 1.
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2.
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| 3.
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In the following exercises, identify the slope and y-intercept of each line.
In the following exercises, graph the line of each equation using its slope and y-intercept.
In the following exercises, determine the most convenient method to graph each line.
In the following exercises, graph and interpret applications of slope and intercept.
- 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 P-intercept 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, per 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.
- 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 S-intercept 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.
In the following exercises, use slopes and y-intercepts to determine if the lines are parallel.
In the following exercises, use slopes and y-intercepts to determine if the lines are perpendicular.
- 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?
Answers
- slope and y-intercept
- slope and y-intercept
- slope and y-intercept
| 9.
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10.
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| 11.
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12.
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| 13.
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14.
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16.
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- horizontal line
- vertical line
- slope–intercept
- intercepts
- slope–intercept
- horizontal line
- intercepts
- slope–intercept
| 25.
a) $28 b) $66.10 c) 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. d) |
26.
a) $15 b) $143 c) The slope, 0.32, means that the cost, C, increases by $0.32 when the number of miles driven, m, increases by 1. The C-intercept means that if Janelle drives 0 miles one day, the cost would be $15. d) |
| 27.
a) $750 b) $2418.60 c) The slope, 0.09, means that Patel’s salary, S, increases by $0.09 for every $1 increase in his sales. The S-intercept means that when his sales are $0, his salary is $750. d) |
28.
a) $2850 b) $4950 c) The slope, 42, means that the cost, C, increases by $42 for when the number of guests increases by 1. The C-intercept means that when the number of guests is 0, the cost would be $750. d) |
- parallel
- parallel
- parallel
- parallel
- parallel
- parallel
- parallel
- parallel
- not parallel
- not parallel
- not parallel
- not parallel
- not parallel
- perpendicular
- perpendicular
- not perpendicular
- not perpendicular
- perpendicular
- perpendicular
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- For every increase of one degree Fahrenheit, the number of chirps increases by four.
- There would be chirps when the Fahrenheit temperature is . (Notice that this does not make sense; this model cannot be used for all possible temperatures.)
Attributions
This chapter has been adapted from “Use the Slope–Intercept Form of an Equation of a Line” in Elementary Algebra (OpenStax) by Lynn Marecek and MaryAnne Anthony-Smith, which is under a CC BY 4.0 Licence. Adapted by Izabela Mazur. See the Adaptation Statement for more information.