Slope of a Line

3.2 Slope of a Line

Topics covered in this section are:

  1. Find the slope of a line
  2. Graph a line given a point and the slope
  3. Graph a line using its slope and intercept
  4. Choose the most convenient method to graph a line
  5. Graph and interpret applications of slope–intercept
  6. Use slopes to identify parallel and perpendicular lines

3.2.1 Find the Slope of a Line

When you graph linear equations, you may notice that some lines tilt up as they go from left to right and some lines tilt down. Some lines are very steep and some lines are flatter.

In mathematics, the measure of the steepness of a line is called the slope of the line.

The concept of slope has many applications in the real world. In construction the pitch of a roof, the slant of the plumbing pipes, and the steepness of the stairs are all applications of slope. and as you ski or jog down a hill, you definitely experience slope.

We can assign a numerical value to the slope of a line by finding the ratio of the rise and run. The rise is the amount the vertical distance changes while the run measures the horizontal change, as shown in this illustration. Slope is a rate of change. See Figure 3.5.

This figure has a diagram of two arrows. The first arrow is vertical and pointed up and labeled “rise”. The second arrow starts at the end of the first. The second arrow is horizontal and pointed right and labeled “run”.
Figure 3.5

SLOPE OF A LINE

The slope of a line is $m=\frac{rise}{run}$.
The rise measures the vertical change and the run measures the horizontal change.

To find the slope of a line, we locate two points on the line whose coordinates are integers. Then we sketch a right triangle where the two points are vertices and one side is horizontal and one side is vertical.

To find the slope of the line, we measure the distance along the vertical and horizontal sides of the triangle. The vertical distance is called the rise and the horizontal distance is called the run.

HOW TO: Find the slope of a line from its graph using $m=\frac{rise}{run}$.

  1. Locate two points on the line whose coordinates are integers.
  2. Starting with one point, sketch a right triangle, going from the first point to the second point.
  3. Count the rise and the run on the legs of the triangle.
  4. Take the ratio of rise to run to find the slope: $m=\frac{rise}{run}$.

Example 1

Find the slope of the line shown.

This figure shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 1 to 9. The y-axis runs from negative 1 to 7. The line goes through the points (0, 5), (3, 3), and (6, 1).
Solution
Locate two points on the graph whose
coordinates are integers.
$(0, 5)$ and $(3, 3)$
Starting at $(0, 5)$, sketch a right triangle to $(3, 3)$ as shown in this graph.
Count the rise— since it goes down, it is negative.The rise is $-2$.
Count the run.The run is $3$.
Use the slope formula.$m=\frac{rise}{run}$
Substitute the values of the rise and run.$m=\frac{-2}{3}$
Simplify.$m=-\frac{2}{3}$
The slope of the line is $-\frac{2}{3}$.
So $y$ decreases by $2$ units as $x$ increases by $3$.

How do we find the slope of horizontal and vertical lines? To find the slope of the horizontal line, $y=4$, we could graph the line, find two points on it, and count the rise and the run. Let’s see what happens when we do this, as shown in the graph below.

The figure then shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 1 to 6. The y-axis runs from negative 1 to 8. The line goes through the points (0, 4) and (3, 4). What is the rise? The rise is 0. What is the run? The run is 3. What is the slope? m equals rise divided by run. m equals 0 divided by 3. m equals 0. The slope of the horizontal line y equals 4 is 0.
What is the rise?The rise is $0$.
What is the run?The run is $3$.
What is the slope?The slope is $m=\frac{rise}{run}$
$m=\frac{0}{3}$
$m=0$
The slope of the horizontal line $y=4$ is $0$.

Let’s also consider a vertical line, the line $x=3$, as shown in the graph.

The figure then shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 2 to 6. The y-axis runs from negative 3 to 3. The line goes through the points (3, 0) and (3, 2). What is the rise? The rise is 2. What is the run? The run is 0. What is the slope? m equals rise divided by run. m equals 2 divided by 0.
What is the rise?The rise is $2$.
What is the run?The run is $0$.
What is the slope?The slope is $m=\frac{rise}{run}$
$m=\frac{2}{0}$

The slope is undefined since division by zero is undefined. So we say that the slope of the vertical line $x=3$ is undefined.

All horizontal lines have slope $0$. When the $y$-coordinates are the same, the rise is $0$.

The slope of any vertical line is undefined. When the $x$-coordinates of a line are all the same, the run is $0$.

SLOPE OF A HORIZONTAL AND VERTICAL LINE 

The slope of a horizontal line, $y=b$, is $0$.
The slope of a vertical line, $x=a$, is undefined.

Example 2

Find the slope of each line:

  • $x=8$
  • $y=-5$
Solution
  • $x=8$
    This is a vertical line. Its slope is undefined.
  • $y=-5$
    This is a horizontal line. It has slope $0$.

QUICK GUIDE TO THE SLOPES OF LINES

The image shows four arrows. The first arrow is slanted and pointing up and to the right and is labeled “positive”. The second arrow is slanted and pointing down and to the right and labeled “negative”. The third arrow is horizontal and labeled “zero”. The fourth arrow is vertical and labeled “undefined”.

Sometimes we’ll need to find the slope of a line between two points when we don’t have a graph to count out the rise and the run. We could plot the points on grid paper, then count out the rise and the run, but as we’ll see, there is a way to find the slope without graphing. Before we get to it, we need to introduce some algebraic notation.

We have seen that an ordered pair $(x, y)$ gives the coordinates of a point. But when we work with slopes, we use two points. How can the same symbol $(x, y)$ be used to represent two different points? Mathematicians use subscripts to distinguish the points.

$(x_1, y_1)$ read “$x$ sub $1$, $y$ sub $1$”
$(x_2, y_2)$ read “$x$ sub $2$, $y$ sub $2$”

We will use $(x_1, y_1)$ to identify the first point and $(x_2, y_2)$ to identify the second point.

If we had more than two points, we could use $(x_3, y_3)$, $(x_4, y_4)$ and so on.

Let’s see how the rise and run relate to the coordinates of the two points by taking another look at the slope of the line between the points $(2, 3)$ and $(7, 6)$, as shown in this graph.

The figure shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 1 to 7. The y-axis runs from negative 1 to 7. The line goes through the points (2, 3) and (7, 6). A right triangle is drawn by connecting the three points (2, 3), (2, 6), and (7, 6). The point (2, 3) is labeled (x 1, y 1). The point (7, 6) is labeled (x 2, y 2). The vertical side of the triangle has labels y 2 minus y 1, 6 minus 3, and 3. The horizontal side of the triangle has labels x 2 minus x 1, 7 minus 2, and 5.
Since we have two points, we will use subscript notation.$(\overset{x_1,}{2}, \overset{y_1}{3}) (\overset{x_2,}{7}, \overset{y_2}{6})$
$m=\frac{rise}{run}$
On the graph, we counted the rise of $3$ and the run of $5$.$m=\frac{3}{5}$
Notice that the rise of $3$ can be found by subtracting the
$y$-coordinates, $6$ and $3$, and the run of $5$ can be found by
subtracting the $x$-coordinates $7$ and $2$.
We rewrite the rise and run by putting in the coordinates.$m=\frac{6-3}{7-2}$
But $6$ is $y_2$, the y-coordinate of the second point
and $3$ is $y_1$, the $y$-coordinate of the first point.
So we can rewrite the slope using subscript notation.
$m=\frac{y_2 – y_1}{7-2}$
Also $7$ is the $x$-coordinate of the second point and $2$ is the $x$-coordinate
of the first point. So again we rewrite the slope using subscript notation.
$m=\frac{y_2 – y_1}{x_2 – x_1}$

We’ve shown that $m=\frac{y_2 – y_1}{x_2 – x_1}$ is really another version of $m=\frac{rise}{run}$. We can use this formula to find the slope of a line when we have two points on the line.

SLOPE OF A LINE BETWEEN TWO POINTS

The slope of the line between two points $(x_1, y_1)$ and $(x_2, y_2)$ is:
$m=\frac{y_2 – y_1}{x_2 – x_1}$

The slope is:

$y$ of the second point minus $y$ of the first point
over
$x$ of the second point minus $x$ of the first point

Example 3

Use the slope formula to find the slope of the line through the points $(-2, -3)$ and $(-7, 4)$.

Solution
We’ll call $(-2, -3)$ point #1 and $(-7, 4)$ point #2. $\begin{pmatrix} x_1, \ y_1 \\ -2, -3 \end{pmatrix}$ $\begin{pmatrix} x_2, \ y_2 \\ -7, \ 4 \end{pmatrix}$
Use the slope formula.$m=\frac{y_2 – y_1}{x_2 – x_1}$
Substitute the values.
$y$ of the second point minus $y$ of the first point.
$x$ of the second point minus $x$ of the first point. $m=\frac{4-(-3)}{-7-(-2)}$
Simplify. $m=\frac{7}{-5}$
$m=-\frac{7}{5}$
Let’s verify this slope on the graph shown.
The figure shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 8 to 2. The y-axis runs from negative 6 to 6. The line goes through the points (negative 7, 4) and (negative 2, negative 3). A right triangle is drawn by connecting the three points (negative 7, 4), (negative 7, negative 3), and (negative 2, negative 3). The vertical side of the triangle is labeled “rise”. The horizontal side of the triangle is labeled “run”.

$m=\frac{rise}{run}$
$m=\frac{7}{-5}$
$m=-\frac{7}{5}$

3.2.2 Graph a Line Given a Point and the Slope

Up to now, in this chapter, we have graphed lines by plotting points, by using intercepts, and by recognizing horizontal and vertical lines.

We can also graph a line when we know one point and the slope of the line. We will start by plotting the point and then use the definition of slope to draw the graph of the line.

Example 4

Graph the line passing through the point $(1, -1)$ whose slope is $m=\frac{3}{4}$.

Solution
Step 1 is to plot the given point. Plot (1, negative 1). The figure then shows the graph of a point on the x y-coordinate plane. The x-axis runs from negative 2 to 6. The y-axis runs from negative 2 to 4. The point (1, negative 1) is plotted and labeled with its coordinates.
Step 2 is to use the slope formula m equals rise divided by run to identify the rise and the run. Identify the rise and the run. m equals 3 divided by 4. Rise divided by run equals 3 divided by 4. Rise equals 3. Run equals 4.
Step 3 is to start at the given point and count out the rise and run to mark the second point. Start at (1, negative 1) and count the rise and the run. Up 3 units, right 4 units. The figure then shows the graph of three points connected by two straight line segments on the x y-coordinate plane. The x-axis runs from negative 2 to 6. The y-axis runs from negative 2 to 4. The points (1, negative 1), (1, 2), and (5, 2) are plotted. A vertical line segment connects (1, negative 1) to (1, 2) and is labeled 3. A horizontal line segment connects (1, 2) to (5, 2) and is labeled 4.
Step 4 is to connect the points with a line. Connect the two points with a line. . The figure then shows the graph of a straight line, three points, and two line segments on the x y-coordinate plane. The x-axis runs from negative 2 to 6. The y-axis runs from negative 2 to 4. The points (1, negative 1), (1, 2), and (5, 2) are plotted. A vertical line segment connects (1, negative 1) to (1, 2). A horizontal line segment connects (1, 2) to (5, 2) and is labeled 4. A straight line is drawn through the points (1, negative 1) and (5, 2) with arrows on both ends.

You can check your work by finding a third point. Since the slope is $m=\frac{3}{4}$, it can also be written as $m=\frac{-3}{-4}$ (negative divided by negative is positive!). Go back to $(1, -1)$ and count out the rise, $-3$, and the run $-4$.

HOW TO: Graph a line given a point and the slope.

  1. Plot the given point.
  2. Use the slope formula $m=\frac{rise}{run}$ to identify the rise and run.
  3. Starting at the given point, count out the rise and run to mark the second point.
  4. Connect the points with a line.

3.2.3 Graph a Line Using its Slope and Intercept

We have graphed linear equations by plotting points, using intercepts, recognizing horizontal and vertical lines, and using one point and the slope of the line. 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 3.6. Let’s look at the graph of the equation $y=\frac{1}{2}x+3$ and find its slope and $y$-intercept.

The figure shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 10 to 10. The y-axis runs from negative 10 to 10. The line goes through the points (0, 3), (2, 4), and (4, 5). A right triangle is drawn by connecting the three points (2, 4), (2, 5), and (4, 5). The vertical side of the triangle is labeled “Rise equals 1”. The horizontal side of the triangle is labeled “Run equals 2”. The line is labeled y equals 1 divided by 2 x plus 3.
Figure 3.6

The red lines in the graph show us the rise is $1$ and the run is $2$. Substituting into the slope formula:

$m=\frac{rise}{run}$
$m=\frac{1}{2}$

The $y$-intercept is $(0, 3)$
Look at the equation of this line.
$y=\textcolor{red}{\frac{1}{2}} x+\textcolor{blue}{3}$
Look at the slope and $y$-intercept.
slope $m=\textcolor{red}{\frac{1}{2}}$ and $y$-intercept $(0, \textcolor{blue}{3})$.
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.”
$m=\textcolor{red}{\frac{1}{2}}$ and $y$-intercept $(0, \textcolor{blue}{3})$
$y=\textcolor{red}{\frac{1}{2}} x+\textcolor{blue}{3}$
$y=\textcolor{red}{m}x + \textcolor{blue}{b}$

SLOPE INTERCEPT FORM OF AN EQUATION OF A LINE

The slope–intercept form of an equation of a line with slope $m$ and $y$-intercept $(0, b)$ is $y=mx+b$.

Let’s practice finding the values of the slope and y-intercept from the equation of a line.

Example 5

Identify the slope and y-intercept of the line from the equation:

  • $y=\frac{-4}{7}x-2$
  • $x+3y=9$
Solution
  • We compare our equation to the slope-intercept form of the equation.
Write the slope–intercept form of the equation of the line.$y=\textcolor{red}{m}x + \textcolor{blue}{b}$
Write the equation of the line.$y=\textcolor{red}{\frac{-4}{7}}x – \textcolor{blue}{2}$
Identify the slope.$m=\textcolor{red}{\frac{-4}{7}}$
Identify the $y$-intercept.$y$-intercept is $(0, \textcolor{blue}{-2})$
  • When an equation of a line is not given in slope–intercept form, our first step will be to solve the equation for $y$.
Solve for $y$.$x+3y=9$
Subtract $x$ from each side.$3y=-x+9$
Divide both side by $3$.$\frac{3y}{3}=\frac{-x+9}{3}$
Simplify.$y=-\frac{1}{3}+3$
Write the slope–intercept form of the equation of the line.$y=\textcolor{red}{m}x + \textcolor{blue}{b}$
Write the equation of the line.$y=\textcolor{red}{-\frac{1}{3}}x – \textcolor{blue}{3}$
Identify the slope.$m=\textcolor{red}{-\frac{1}{3}}$
Identify the $y$-intercept.$y$-intercept is $(0, \textcolor{blue}{3})$

We have graphed a line using the slope and a point. Now that we know how to find the slope and $y$-intercept of a line from its equation, we can use the $y$-intercept as the point, and then count out the slope from there.

Example 5

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

Solution
$y=mx+b$
The equation is in slope–intercept form.$y=-x+4$
Identify the slope and $y$-intercept.$m=-1$
$y$-intercept is $(0, 4)$
Plot the $y$-intercept.See the graph.
Identify the rise over run.$m=\frac{-1}{1}$
Count out the rise and run to mark the second point.rise is $-1$, run is $1$
The graph of the line y=-x+4. The rise is -1 and the run is 1. The slope m=-1.

Draw the line as shown in the graph.

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.

The table has a title row that reads “Methods to Graph Lines”. Below this are four columns. The first column contains the following: Point Plotting. A blank table with two columns and four rows. The first row is a header row with the headers “x” and “y”. Find three points. Plot the points, make sure they line up, them draw the line. The second column contains: Slope-Intercept. Y equals m x plus b. Find the slope and y-intercept, then count the slope to get a second point. The third column: Intercepts. A table with two columns and four rows. The first row is a header row with the headers “x” and “y”. In the first row there is a 0 in the x column. In the second row there is a 0 in the y column. The remaining spaces are blank. Fourth column. Recognize vertical and horizontal lines. The equation has only one variable. X equals a vertical. Y equals b horizontal.

3.2.4 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. Let’s look for some patterns to help determine the most convenient method to graph a line.

Here are five equations we graphed in this chapter, and the method we used to graph each of them.

EquationMethod
#1$x=2$Vertical line
#2$y=-1$Horizontal line
#3$-x+2y=6$Intercepts
#4$4x-3y=12$Intercepts
#5$y=-x+4$Slope-intercept

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 $y$.

Equation #$5$ is 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.
    • $x=a$ is a vertical line passing through the $x$-axis at $a$.
    • $y=b$ is a horizontal line passing through the $y$-axis at $b$.
  • If $y$ is isolated on one side of the equation, in the form $y=mx+b$, graph by using the slope and $y$-intercept.
    • Identify the slope and $y$-intercept then graph.
  • If the equation is of the form $Ax+By=C$, find the intercepts.
    • Find the $x$- and $y$-intercepts, a third point, and then graph.

Example 6

Determine the most convenient method to graph each line:

  • $y=5$
  • $4x-5y=20$
  • $x=-3$
  • $y=-\frac{5}{9}x+8$
Solution
  • $y=5$
    This equation has only one variable, y. Its graph is a horizontal line crossing the $y$-axis at $5$.
  • $4x-5y=20$
    The equation is of the form $Ax+By=C$. The easiest way to graph it will be to find the intercepts and one more point.
  • $x=-3$
    There is only one variable, $x$. The graph is a vertical line crossing the $x$-axis at $-3$.
  • $y=-\frac{5}{9}x+8$
    Since this equation is in $y=mx+b$ form, it will be easiest to graph this line by using the slope and $y$-intercepts.

3.2.5 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 uses real-world data, different letters are used for the variables, instead of using only $x$ and $y$. The variable names remind us of what quantities are being measured.

Also, we often will need to extend the axes in our rectangular coordinate system to bigger positive and negative numbers to accommodate the data in the application.

Example 7

The equation $F=\frac{9}{5}C+32$ is used to convert temperatures, $C$, on the Celsius scale to temperatures, $F$, 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 $F$-intercept of the equation.
  • Graph the equation.
Solution
  • Part 1
Find the Fahrenheit temperature for a Celsius temperature of $0$.$F=\frac{9}{5}C+32$
Find $F$ when $C=0$.$F=\frac{9}{5}(0)+32$
Simplify.$F=32$
  • Part 2
Find the Fahrenheit temperature for a Celsius temperature of $20$.$F=\frac{9}{5}C+32$
Find $F$ when $C=20$.$F=\frac{9}{5}(20)+32$
Simplify.$F=36+32$
Simplify.$F=68$
  • Part 3
    Interpret the slope and $F$-intercept of the equation.
    Even though this equation uses $F$ and $C$, it is still in slope–intercept form.
    $\textcolor{red}{y}=m\textcolor{blue}{x}+b$
    $\textcolor{red}{F}=m\textcolor{blue}{C}+b$
    $F=\frac{9}{5}C+32$
    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 $0º$ on the Celsius scale, it is $32º$ on the Fahrenheit scale.
  • Part 4 – Graph the equation
    We’ll need to use a larger scale than our usual. Start at the $F$-intercept $(0, 32)$, and then count out the rise of $9$ and run of $5$ to get a second point as shown in the graph.
This figure shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 40 to 80. The y-axis runs from negative 40 to 80. The line goes through the points (0, 32) and (5, 41).

The cost of running some types of 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 8

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.
Solution
  • Part 1
Find Sam’s cost for a week when he drives $0$ miles.$C=0.5m+60$
Find $C$ when $m=0$.$C=0.5(0)+60$
Simplify.$C=60$
Sam’s costs are $\$60$ when he drives $0$ miles.
  • Part 2
Find Sam’s cost for a week when he drives $250$ miles.$C=0.5m+60$
Find $C$ when $m=250$.$C=0.5(250)+60$
Simplify.$C=185$
Sam’s costs are $\$185$ when he drives $250$ miles.
  • Part 3 – Interpret the slope and $C$-intercept of the equation.
    $\textcolor{red}{y} = m\textcolor{blue}{x}+b$
    $\textcolor{red}{C} = 0.5\textcolor{blue}{m}+60$
    The slope, $0.5$, means that the weekly cost, $C$, increases by $\$0.50$ when the number of miles driven, $m$, increases by $1$.
    The $C$-intercept means that when the number of miles driven is $0$, the weekly cost is $\$60$.
  • Part 4- Graph the equation
    We’ll need to use a larger scale than our usual. Start at the $C$-intercept $(0, 60)$.
    To count out the slope $m= 0.5$, we rewrite it as an equivalent fraction that will make our graphing easier.
$m=0.5$
Rewrite as a fraction.$m=\frac{0.5}{1}$
Multiply numerator and denominator by $100$.$m=\frac{0.5(100)}{1(100)}$
Simplify.$m=\frac{50}{100}$

So to graph the next point go up $50$ from the intercept of $60$ and then to the right $100$. The second point will be $(100, 110)$.

This figure shows the graph of a straight line on the x y-coordinate plane. The x-axis runs from negative 1 to 350. The y-axis runs from negative 1 to 350. The line goes through the points (0, 60) and (200, 160).

3.2.6 Use Slopes to Identify Parallel and Perpendicular Lines

Two lines that have the same slope are called parallel lines. Parallel lines have the same steepness and 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 3.7.

This figure shows the graph of a two straight lines on the x y-coordinate plane. The x-axis runs from negative 8 to 8. The y-axis runs from negative 8 to 8. The first line goes through the points (0, 3) and (5, 5). The second line goes through the points (0, negative 2) and (5, 0). The lines are parallel meaning they will always be the same distance apart and never intersect. They are slanted by the same angle.
Figure 3.7

Verify that both lines have the same slope, $m=\frac{2}{5}$, 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, like the lines shown in this graph.

This figure shows the graph of a two straight vertical line on the x y-coordinate plane. The x-axis runs from negative 8 to 8. The y-axis runs from negative 8 to 8. The first line goes through the points (2, 0) and (2, 1). The second line goes through the points (5, 0) and (5, 1). The lines are parallel meaning they will always be the same distance apart and never intersect.
Figure 3.8

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 $m_1$ and $m_2$ are the slopes of two parallel lines then $m_1=m_2$.
  • Parallel vertical lines have different $x$-intercepts.

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 9

Use slopes and $y$-intercepts to determine if the lines are parallel:

  • $3x-2y=6$ and $y=\frac{3}{2}x+1$
  • $y=2x-3$ and $-6x+3y=-9$
Solution

Part 1

$3x-2y=6$and$y=\frac{3}{2}x+1$
Solve the first equation for $y$.$-2y=6-3x$
$y=\frac{6-3x}{-2}$
The equation is now in slope-
intercept form.
$y=\frac{3}{2}x-3$
The equation of the second line
is already in slope-intercept form.
$y=\frac{3}{2}x+1$
Identify the slope and $y$-intercept
of both lines.
$y=\frac{3}{2}x-3$
$y=mx+b$
$m=\frac{3}{2}$
$y=\frac{3}{2}x+1$
$y=mx+b$
$m=\frac{3}{2}$
$y$-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.

Part 2

$y=2x-3$and$-6x+3y=-9$
The first equation is already in
slope-intercept form.
$y=2x-3$
Solve the second equation for $y$.$\begin{align*}-6x+3y&=-9 \\ 3y&=6x-9 \\ \frac{3y}{3}&=\frac{6x-9}{3} \\ y&=2x-3 \end{align*}$
The second equation is now in slope-intercept form.$y=2x-3$
Identify the slope and $y$-intercept
of both lines.
$y=2x-3$
$y=mx+b$
$m=2$
$y=2x-3$
$y=mx+b$
$m=2$
$y$-intercept is $(0, -3)$$y$-intercept is $(0, -3)$

The lines have the same slope, but they also have the same $y$-intercepts. Their equations represent the same line and we say the lines are coincident. They are not parallel; they are the same line.

Example 10

Use slopes and y-intercepts to determine if the lines are parallel:

  • $y=-4$ and $y=3$
  • $x=-2$ and $x=-5$
Solution
  • $y=-4$ and $y=3$
    We recognize right away from the equations that these are horizontal lines, and so we 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.
  • $x=-2$ and $x=-5$
    We recognize right away from the equations that these are vertical lines, and so we know their slopes are undefined.
    Since the vertical lines cross the $x$-axis at $x=-2$ and $x=-5$, we know the $y$-intercepts are $(-2, 0)$ and $(-5, 0)$.
    The lines are vertical and have different $x$-intercepts and so they are parallel.

Let’s look at the lines whose equations are $y=\frac{1}{4}x-1$ and $y=-4x+2$, shown in Figure 3.9.

This figure shows the graph of a two perpendicular straight lines on the x y-coordinate plane. The x-axis runs from negative 8 to 8. The y-axis runs from negative 8 to 8. The first line goes through the points (0, negative 1) and (4, 0). The first line is labeled y equals 1 divided by 4 x minus 1. The second line goes through the points (0, 2) and (1, negative 2). The second line is labeled y equals negative 4 x plus 2. The lines are perpendicular meaning they form a right angle where they intersect.
Figure 3.9

These lines lie in the same plane and intersect in right angles. We call these lines perpendicular.

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 $-1$.

$m_1 \cdot m_2$
$\frac{1}{4}(-4)$
$-1$

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 $m_1$ and $m_2$ are the slopes of two perpendicular lines, then:
    • their slopes are negative reciprocals of each other, $m_1=-\frac{1}{m_2}$.
    • the product of their slopes is $-1$, $m_1 \cdot m_2=-1$.
  • A vertical line and a horizontal line 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 opposite reciprocals. If the product of the slopes is $−1$, the lines are perpendicular.

Example 11

Use slopes to determine if the lines are perpendicular:

  • $y=-5x-4$ and $x-5y=5$
  • $7x+2y=3$ and $2x+7y=5$
Solution
  • Part 1
The first equation is in
slope-intercept form.
$y=-5x-4$
Solve the second equation for $y$.$\begin{align*} x-5y&=5\\ -5y&=-x+5 \\ \frac{-5y}{-5}&=\frac{-x+5}{-5} \\ y&=\frac{1}{5}x-1 \end{align*}$
Identify the slope of each line.$y=-5x-4$
$y=mx+b$
$m_1=-5$
$y=\frac{1}{5}x-1$
$y=mx+b$
$m_2=\frac{1}{5}$

The slopes are negative reciprocals of each other, so the lines are perpendicular. We check by multiplying the slopes. Since $-5(\frac{1}{5})=-1$, it checks.

  • Part 2
Solve the equations for $y$.$\begin{align*} 7x+2y&=3\\ 2y&=-7x+3 \\ \frac{2y}{2}&=\frac{-7x+3}{2} \\ y&=-\frac{7}{2}x+\frac{3}{2} \end{align*}$$\begin{align*} 2x+7y&=5\\ 7y&=-2x+5 \\ \frac{7y}{7}&=\frac{-2x+5}{7} \\ y&=-\frac{2}{7}x+\frac{5}{7} \end{align*}$
Identify the slope of each line.$y=mx+b$
$m_1=-\frac{7}{2}$
$y=mx+b$
$m_2=-\frac{2}{7}$

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

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