4.5 Use the Slope-Intercept Form of an Equation of a Line

Learning objectives

Learning objectives

• 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.

In Graph Linear Equations in Two Variables, we graphed the line of the equation $y=\frac{1}{2}x+3$ by plotting points. See Figure 4.24. 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 $(0,3)$. The equation of this line is:

Notice, the line has:

When a linear equation is solved for $y$, the coefficient of the $x$ term is the slope and the constant term is the y-coordinate of the y-intercept. We say that the equation $y=\frac{1}{2}x+3$ is in slope–intercept form.

Sometimes the slope–intercept form is called the “y-form.”

Example 4.40

Solution

To find the slope of the line, we need to choose two points on the line. We’ll use the points $(0,1)$ and $(1,3)$.

The slope is the same as the coefficient of $x$ and the y-coordinate of the y-intercept is the same as the constant term.

Identify the Slope and y-Intercept 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 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 4.41

Solution

We compare our equation to the slope–intercept form of the equation.

When an equation of a line is not given in slope–intercept form, our first step will be to solve the equation for $y$.

Example 4.42

Solution

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$y$.

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 4.43

Example 4.44

Solution

Example 4.45

Solution

Example 4.46

Solution

We have used a grid with $x$ and $y$ both going from about $-10$ 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 4.47

Solution

We’ll use a grid with the axes going from about $-80$ to 80.

Now that we have graphed lines by using the slope and y-intercept, let’s summarize all the methods we have used to graph lines. See Figure 4.25.

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 $x$ and $y$ are on the same side of the equation. These two equations are of the form $Ax+By=C$. We substituted $y=0$ to find the x-intercept and $x=0$ to find the y-intercept, and then found a third point by choosing another value for $x$ or $y$.

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.

Example 4.48

Solution

1. ⓐ $y=-6$ This equation has only one variable,$y$. Its graph is a horizontal line crossing the y-axis at $-6$.

2. ⓑ $5x-3y=15$ This equation is of the form $Ax+By=C$. The easiest way to graph it will be to find the intercepts and one more point.

3. ⓒ $x=7$ There is only one variable, $x$. The graph is a vertical line crossing the x-axis at 7.

4. ⓓ $y=\frac{2}{5}x-1$ Since this equation is in $y=mx+b$ form, it will be easiest to graph this line by using the slope and y-intercept.

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 4.49

Solution

Table

4.41

ⓒ Interpret the slope and F-intercept of the equation.

Even though this equation uses $F$and $C$, it is still in slope–intercept form.

The slope, $\frac{9}{5}$, 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 $0^{\circ}$ on the Celsius scale, it is $32^{\circ}$ on the Fahrenheit scale.

ⓓ Graph the equation.

We’ll need to use a larger scale than our usual. Start at the F-intercept $(0,32)$ then count out the rise of 9 and the run of 5 to get a second point. See Figure 4.26.

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 4.50

Solution

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 4.27.

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 4.28.

Let’s graph the equations $y=-2x+3$ and $2x+y=-1$ on the same grid. The first equation is already in slope–intercept form: $y=-2x+3$. We solve the second equation for $y$:

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 4.51

Solution

Table

4.42

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.

Example 4.52

Solution

Table

4.43

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 $y=-4$ and at $y=3$, we know the y-intercepts are $(0,-4)$ and $(0,3)$. The lines have the same slope and different y-intercepts and so they are parallel.

Example 4.53

Solution

Since there is no$y$, the equations cannot be put in slope–intercept form. But we recognize them as equations of vertical lines. Their x-intercepts are $-2$ and $-5$. Since their x-intercepts are different, the vertical lines are parallel.

Example 4.54

Solution

Table

4.44

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.

Use Slopes to Identify Perpendicular Lines

Let’s look at the lines whose equations are $y=\frac{1}{4}x-1$ and $y=-4x+2$, shown in Figure 4.29.

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 $y=\frac{1}{4}x-1$ rises, so its slope is positive. The line$y=-4x+2$ 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, ${m}_{1}=\frac{1}{4}$, and the slope of the second line, ${m}_{2}=-4$, we can see that they are negative reciprocals of each other. If we multiply them, their product is $-1.$

This is always true for perpendicular lines and leads us to this definition.

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 $-1$, the lines are perpendicular. Perpendicular lines may have the same y-intercepts.

Example 4.55

Solution

The slopes are negative reciprocals of each other, so the lines are perpendicular. We check by multiplying the slopes,

Example 4.56

Solution

Table

4.45

The slopes are reciprocals of each other, but they have the same sign. Since they are not negative reciprocals, the lines are not perpendicular.

Section 4.5 Exercises

Section 4.5 Exercises

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 y-intercept of each line. Compare the values to the equation $y=mx+b$.

Identify the Slope and y-Intercept From an Equation of a Line

In the following exercises, identify the slope and y-intercept 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 y-intercept.

Choose the Most Convenient Method to Graph a Line

In the following exercises, determine the most convenient method to graph each line.

Graph and Interpret Applications of Slope–Intercept

Use Slopes to Identify Parallel Lines

In the following exercises, use slopes and y-intercepts to determine if the lines are parallel.

Use Slopes to Identify Perpendicular Lines

In the following exercises, use slopes and y-intercepts to determine if the lines are perpendicular.

Everyday Math

Writing Exercises

ⓐ 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 well-prepared for the next section? Why or why not?

1. Use the graph to find the slope and y-intercept of the line $y=\frac{2}{3}x-1$. Compare these values to the equation $y=mx+b$.

2. Use the graph to find the slope and y-intercept of the line $y=\frac{1}{2}x+3$. Compare these values to the equation $y=mx+b$.

3. Identify the slope and y-intercept of the line $y=\frac{2}{5}x-1$.

4. Identify the slope and y-intercept of the line $y=-\frac{4}{3}x+1$.

5. Identify the slope and y-intercept of the line $x+4y=8$.

6. Identify the slope and y-intercept of the line $3x+2y=12$.

1. Graph the line of the equation $y=4x+1$ using its slope and y-intercept.

2. Graph the line of the equation $y=2x-3$ using its slope and y-intercept.

3. Graph the line of the equation $y=-x-3$ using its slope and y-intercept.

4. Graph the line of the equation $y=-x-1$ using its slope and y-intercept.

5. Graph the line of the equation $y=-\frac{5}{2}x+1$ using its slope and y-intercept.

6. Graph the line of the equation $y=-\frac{3}{4}x-2$ using its slope and y-intercept.

1. Graph the line of the equation $2x-y=6$ using its slope and y-intercept.

2. Graph the line of the equation $3x-2y=8$ using its slope and y-intercept.

3. Graph the line of the equation $y=0.5x+25$ using its slope and y-intercept.

4. Graph the line of the equation $y=0.1x-30$ using its slope and y-intercept.

5. Determine the most convenient method to graph each line: ⓐ $3x+2y=12$ ⓑ $y=4$ ⓒ $y=\frac{1}{5}x-4$ ⓓ $x=-7$.

6. Determine the most convenient method to graph each line: ⓐ $x=6$ ⓑ $y=-\frac{3}{4}x+1$ ⓒ $y=-8$ ⓓ $4x-3y=-1$.

1. The equation $h=2s+50$ is used to estimate a woman’s height in inches, h, based on her shoe size, s.

1. ⓐ Estimate the height of a child who wears women’s shoe size 0.

2. ⓑ Estimate the height of a woman with shoe size 8.

3. ⓒ Interpret the slope and h-intercept of the equation.

4. ⓓ Graph the equation.

2. The equation $T=\frac{1}{4}n+40$ is used to estimate the temperature in degrees Fahrenheit, T, based on the number of cricket chirps, n, in one minute.

1. ⓐ Estimate the temperature when there are no chirps.

2. ⓑ Estimate the temperature when the number of chirps in one minute is 100.

3. ⓒ Interpret the slope and T-intercept of the equation.

4. ⓓ Graph the equation.

3. Sam drives a delivery van. The equation $C=0.5m+60$ 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.

4. Loreen has a calligraphy business. The equation $C=1.8n+35$ models the relation between her weekly cost, C, in dollars and the number of wedding invitations, n, that she writes.

1. ⓐ Find Loreen’s cost for a week when she writes no invitations.

2. ⓑ Find the cost for a week when she writes 75 invitations.

3. ⓒ Interpret the slope and C-intercept of the equation.

4. ⓓ Graph the equation.

5. Use slopes and y-intercepts to determine if the lines $2x+5y=5\,\mathrm{and}\,y=-\frac{2}{5}x-4$ are parallel.

6. Use slopes and y-intercepts to determine if the lines $4x-3y=6\,\mathrm{and}\,y=\frac{4}{3}x-1$ are parallel.

1. Use slopes and y-intercepts to determine if the lines $y=8\,\mathrm{and}\,y=-6$ are parallel.

2. Use slopes and y-intercepts to determine if the lines $y=1\,\mathrm{and}\,y=-5$ are parallel.

3. Use slopes and y-intercepts to determine if the lines $x=1$ and $x=-5$ are parallel.

4. Use slopes and y-intercepts to determine if the lines $x=8$ and $x=-6$ are parallel.

5. Use slopes and y-intercepts to determine if the lines $y=-\frac{1}{2}x-1$ and $x+2y=2$ are parallel.

6. Use slopes and y-intercepts to determine if the lines $y=\frac{3}{4}x-3$ and $3x-4y=12$ are parallel.

1. Use slopes to determine if the lines $y=-3x+2$ and $x-3y=4$ are perpendicular.

2. Use slopes to determine if the lines $y=2x-5$ and $x+2y=-6$ are perpendicular.

3. Use slopes to determine if the lines $5x+4y=1$ and $4x+5y=3$ are perpendicular.

4. Use slopes to determine if the lines $2x-9y=3$ and $9x-2y=1$ are perpendicular.

5. $y=4x-2$

6. $y=-3x+1$

1. $y=-\frac{2}{5}x+3$

2. $y=-9x+7$

3. $y=4x-10$

4. $4x+y=8$

5. $8x+3y=12$

6. $7x-3y=9$

1. $y=x+4$

2. $y=2x-3$

3. $y=-x+3$

4. $y=-x-2$

5. $y=-\frac{2}{5}x-3$

6. $y=-\frac{2}{3}x+1$

1. $4x-3y=6$

2. $y=0.3x+25$

3. $y=4$

4. $x=-3$

5. $y=-5x+2$

6. $x-y=1$

1. $y=\frac{4}{5}x-3$

2. $y=-1$

3. $2x-5y=-10$

4. $y=-\frac{1}{3}x+5$

5. The equation $P=28+2.54w$ 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.

1. ⓐ Find the payment for a month when Randy used 0 units of water.

2. ⓑ Find the payment for a month when Randy used 15 units of water.

3. ⓒ Interpret the slope and P-intercept of the equation.

4. ⓓ Graph the equation.

6. Janelle is planning to rent a car while on vacation. The equation $C=0.32m+15$ models the relation between the cost in dollars, C, per day and the number of miles, m, she drives in one day.

1. ⓐ Find the cost if Janelle drives the car 0 miles one day.

2. ⓑ Find the cost on a day when Janelle drives the car 400 miles.

3. ⓒ Interpret the slope and C–intercept of the equation.

4. ⓓ Graph the equation.

1. Patel’s weekly salary includes a base pay plus commission on his sales. The equation $S=750+0.09c$ models the relation between his weekly salary, S, in dollars and the amount of his sales, c, in dollars.

1. ⓐ Find Patel’s salary for a week when his sales were 0.

2. ⓑ Find Patel’s salary for a week when his sales were 18,540.

3. ⓒ Interpret the slope and S-intercept of the equation.

4. ⓓ Graph the equation.

2. Margie is planning a dinner banquet. The equation $C=750+42g$ models the relation between the cost in dollars, C of the banquet and the number of guests, g.

1. ⓐ Find the cost if the number of guests is 50.

2. ⓑ Find the cost if the number of guests is 100.

3. ⓒ Interpret the slope and C–intercept of the equation.

4. ⓓ Graph the equation.

3. $y=\frac{2}{3}x-1;\,2x-3y=-2$

4. $3x-4y=-2;\,y=\frac{3}{4}x-3\,$

5. $6x-3y=9;\,2x-y=3$

6. $8x+6y=6;\,12x+9y=12$

1. $x=7;\,x=-8$

2. $x=-3;\,x=-2$

3. $y=5;\,y=1$

4. $y=-1;\,y=2$

5. $4x+4y=8;\,x+y=2$

6. $5x-2y=11;\,5x-y=7$

1. $4x-8y=16;\,x-2y=4$

2. $x-5y=10;\,5x-y=-10$

3. $9x-5y=4;\,5x+9y=-1$

4. $x-4y=8;4x+y=2$

5. $2x+3y=5;3x-2y=7$

6. $3x-4y=8;4x-3y=6$

1. $2x+4y=3;6x+3y=2$

2. $2x-6y=4;12x+4y=9$

3. $8x-2y=7;3x+12y=9$

4. The equation $n=4T-160$ is used to estimate the number of cricket chirps, n, in one minute based on the temperature in degrees Fahrenheit, T.

1. ⓐ Explain what the slope of the equation means.

2. ⓑ Explain what the n–intercept of the equation means. Is this a realistic situation?

5. Why are all horizontal lines parallel?