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Graphing Linear Equations in Two Variables

Module by: Wade Ellis, Denny Burzynski. E-mail the authors

Summary: This module is from Elementary Algebra by Denny Burzynski and Wade Ellis, Jr. In this chapter the student is shown how graphs provide information that is not always evident from the equation alone. The chapter begins by establishing the relationship between the variables in an equation, the number of coordinate axes necessary to construct its graph, and the spatial dimension of both the coordinate system and the graph. Interpretation of graphs is also emphasized throughout the chapter, beginning with the plotting of points. The slope formula is fully developed, progressing from verbal phrases to mathematical expressions. The expressions are then formed into an equation by explicitly stating that a ratio is a comparison of two quantities of the same type (e.g., distance, weight, or money). This approach benefits students who take future courses that use graphs to display information. The student is shown how to graph lines using the intercept method, the table method, and the slope-intercept method, as well as how to distinguish, by inspection, oblique and horizontal/vertical lines. Objectives of this module: be able to relate solutions to a linear equation to lines, know the general form of a linear equation, be able to construct the graph of a line using the intercept method, be able to distinguish, by their equations, slanted, horizontal, and vertical lines.

Overview

  • Solutions and Lines
  • General form of a Linear Equation
  • The Intercept Method of Graphing
  • Graphing Using any Two or More Points
  • Slanted, Horizontal, and Vertical Lines

Solutions and Lines

We know that solutions to linear equations in two variables can be expressed as ordered pairs. Hence, the solutions can be represented by point in the plane. We also know that the phrase “graph the equation” means to locate the solution to the given equation in the plane. Consider the equation y−2x=−3 y−2x=−3 . We’ll graph six solutions (ordered pairs) to this equation on the coordinates system below. We’ll find the solutions by choosing x-values x-values (from −1 to +4 −1 to +4 ), substituting them into the equation y−2x=−3 y−2x=−3 , and then solving to obtain the corresponding y-values y-values . We can keep track of the ordered pairs by using a table.

y−2x=−3 y−2x=−3

Table 1
If x= x= Then y= y= Ordered Pairs
−1 −1 −5 −5 ( −1, −5 ) ( −1, −5 )
0 −3 −3 ( 0, −3 ) ( 0, −3 )
1 −1 −1 ( 1, −1 ) ( 1, −1 )
2 1 ( 2, 1 ) ( 2, 1 )
3 3 ( 3, 3 ) ( 3, 3 )
4 5 ( 4, 5 ) ( 4, 5 )

Total six points plotted for graphing the line y minus two x equals negative three. The coordinates of these points are negative one, negative five; zero, negative three; one, negative one; two, one; three, three; and four, five.

We have plotted only six solutions to the equation y−2x=−3 y−2x=−3 . There are, as we know, infinitely many solutions. By observing the six points we have plotted, we can speculate as to the location of all the other points. The six points we plotted seem to lie on a straight line. This would lead us to believe that all the other points (solutions) also lie on that same line. Indeed, this is true. In fact, this is precisely why first-degree equations are called linear equations.

Linear Equations Produce Straight Lines

Line ↕ Linear Line ↕ Linear

A graph of a line passing through six points with the coordinates negative one, negative five; zero, negative three; one, negative one; two, one; three, three; and four, five.

General Form of a Linear Equation

General Form of a Linear Equation in Two Variables

There is a standard form in which linear equations in two variables are written. Suppose that a a , b b , and c c are any real numbers and that a a and b b cannot both be zero at the same time. Then, the linear equation in two variables

ax+by=c ax+by=c

is said to be in general form.

We must stipulate that a a and b b cannot both equal zero at the same time, for if they were we would have

0x+0y=c 0x+0y=c

0=c 0=c

This statement is true only if c=0 c=0 . If c c were to be any other number, we would get a false statement.

Now, we have the following:

The graphing of all ordered pairs that solve a linear equation in two variables produces a straight line.

This implies,

The graph of a linear equation in two variables is a straight line.

From these statements we can conclude,

If an ordered pair is a solution to a linear equations in two variables, then it lies on the graph of the equation.

Also,

Any point (ordered pairs) that lies on the graph of a linear equation in two variables is a solution to that equation.

The Intercept Method of Graphing

When we want to graph a linear equation, it is certainly impractical to graph infinitely many points. Since a straight line is determined by only two points, we need only find two solutions to the equation (although a third point is helpful as a check).

Intercepts

When a linear equation in two variables is given in general from, ax+by=c ax+by=c , often the two most convenient points (solutions) to fine are called the Intercepts: these are the points at which the line intercepts the coordinate axes. Of course, a horizontal or vertical line intercepts only one axis, so this method does not apply. Horizontal and vertical lines are easily recognized as they contain only one variable. (See Sample Set C C .)

A graph of a line sloped down and to the right. The line intersects the x axis at a positive value of x, and the y axis at a positive value of y. The points where the line intersects the axes are labeled x-intercept and y-intercept respectively.

y-Intercept y-Intercept

The point at which the line crosses the y-axis y-axis is called the y-intercept y-intercept . The x-value x-value at this point is zero (since the point is neither to the left nor right of the origin).

x-Intercept x-Intercept

The point at which the line crosses the x-axis x-axis is called the x-intercept x-intercept  and the y-value y-value at that point is zero. The y-intercept y-intercept can be found by substituting the value 0 for x x into the equation and solving for y y . The x-intercept x-intercept can be found by substituting the value 0 for y y into the equation and solving for x x .

Intercept Method

Since we are graphing an equation by finding the intercepts, we call this method the intercept method

Sample Set A

Graph the following equations using the intercept method.

Example 1

y−2x=−3 y−2x=−3

To find the y-intercept y-intercept , let x=0 x=0 and y=b y=b .

b−2( 0 ) = −3 b−0 = −3 b = −3 b−2( 0 ) = −3 b−0 = −3 b = −3

Thus, we have the point ( 0, −3 ) ( 0, −3 ) . So, if x=0 x=0 , y=b=−3 y=b=−3 .

To find the x-intercept x-intercept , let y=0 y=0 and x=a x=a .

0−2a = −3 −2a = −3 Divide by -2. a = −3 −2 a = 3 2 0−2a = −3 −2a = −3 Divide by -2. a = −3 −2 a = 3 2

Thus, we have the point ( 3 2 ,0 ) ( 3 2 ,0 ) . So, if x=a= 3 2 x=a= 3 2 , y=0 y=0 .

Construct a coordinate system, plot these two points, and draw a line through them. Keep in mind that every point on this line is a solution to the equation y−2x=−3 y−2x=−3 .

A graph of a line passing through two points with coordinates zero, negative three and three over two, zero.

Example 2

−2x+3y=3 −2x+3y=3

To find the y-intercept y-intercept , let x=0 x=0 and y=b y=b .

−2( 0 )+3b = 3 0+3b = 3 3b = 3 b = 1 −2( 0 )+3b = 3 0+3b = 3 3b = 3 b = 1

Thus, we have the point ( 0, 1 ) ( 0, 1 ) . So, if x=0 x=0 , y=b=1 y=b=1 .

To find the x-intercept x-intercept , let y=0 y=0 and x=a x=a .

−2a+3( 0 ) = 3 −2a+0 = 3 −2a = 3 a = 3 −2 a = − 3 2 −2a+3( 0 ) = 3 −2a+0 = 3 −2a = 3 a = 3 −2 a = − 3 2

Thus, we have the point ( − 3 2 , 0 ) ( − 3 2 , 0 ) . So, if x=a=− 3 2 x=a=− 3 2 , y=0 y=0 .

Construct a coordinate system, plot these two points, and draw a line through them. Keep in mind that all the solutions to the equation −2x+3y=3 −2x+3y=3 are precisely on this line.

A graph of a line passing through two points with coordinates zero, one and negative three over two, zero.

Example 3

4x+y=5 4x+y=5

To find the y-intercept y-intercept , let x=0 x=0 and y=b y=b .

4( 0 )+b = 5 0+b = 5 b = 5 4( 0 )+b = 5 0+b = 5 b = 5

Thus, we have the point ( 0, 5 ) ( 0, 5 ) . So, if x=0 x=0 , y=b=5 y=b=5 .

To find the x-intercept x-intercept , let y=0 y=0 and x=a x=a .

4a+0 = 5 4a = 5 a = 5 4 4a+0 = 5 4a = 5 a = 5 4

Thus, we have the point ( 5 4 , 0 ) ( 5 4 , 0 ) . So, if x=a= 5 4 x=a= 5 4 , y=0 y=0 .

Construct a coordinate system, plot these two points, and draw a line through them.

A graph of a line passing through two points with coordinates zero, five and five over four, zero.

Practice Set A

Exercise 1

Graph 3x+y=3 3x+y=3 using the intercept method.

An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

When x=0 x=0 , y=3 y=3 ; when y=0 y=0 , x=1 x=1

A graph of a line passing through two points with coordinates zero, three and one, zero.

Graphing Using any Two or More Points

The graphs we have constructed so far have been done by finding two particular points, the intercepts. Actually, any two points will do. We chose to use the intercepts because they are usually the easiest to work with. In the next example, we will graph two equations using points other than the intercepts. We’ll use three points, the extra point serving as a check.

Sample Set B

Example 4

x−3y=−10 x−3y=−10 .
We can find three points by choosing three x-values x-values and computing to find the corresponding y-values y-values . We’ll put our results in a table for ease of reading.

Since we are going to choose x-values x-values and then compute to find the corresponding y-values y-values , it will be to our advantage to solve the given equation for y y .

x−3y = −10 Subtract  x  from both sides. −3y = −x−10 Divide both sides by -3. y = 1 3 x+ 10 3 x−3y = −10 Subtract  x  from both sides. −3y = −x−10 Divide both sides by -3. y = 1 3 x+ 10 3

Table 2
x x y y ( x, y ) ( x, y )
1 If x=1 x=1 , then y= 1 3 ( 1 )+ 10 3 = 1 3 + 10 3 = 11 3 y= 1 3 ( 1 )+ 10 3 = 1 3 + 10 3 = 11 3 ( 1,  11 3 ) ( 1,  11 3 )
−3 −3 If x=−3 x=−3 , then y= 1 3 ( −3 )+ 10 3 =−1+ 10 3 = 7 3 y= 1 3 ( −3 )+ 10 3 =−1+ 10 3 = 7 3 ( −3, 7 3 ) ( −3, 7 3 )
3 If x=3 x=3 , then y= 1 3 ( 3 )+ 10 3 =1+ 10 3 = 13 3 y= 1 3 ( 3 )+ 10 3 =1+ 10 3 = 13 3 ( 3,  13 3 ) ( 3,  13 3 )

Thus, we have the three ordered pairs (points), ( 1,  11 3 ) ( 1,  11 3 ) , ( −3,  7 3 ) ( −3,  7 3 ) , ( 3,  13 3 ) ( 3,  13 3 ) . If we wish, we can change the improper fractions to mixed numbers, ( 1,3  2 3 ) ( 1,3  2 3 ) , ( −3,2  1 3 ) ( −3,2  1 3 ) , ( 3,4  1 3 ) ( 3,4  1 3 ) .

A graph of a line passing through three points with coordinates negative three, seven over three; one, eleven over three; and three, thirteen over three.

Example 5

4x+4y=0 4x+4y=0

We solve for y y .

4y = −4x y = −x 4y = −4x y = −x

Table 3
x x y y ( x, y ) ( x, y )
0 0 ( 0, 0 ) ( 0, 0 )
2 −2 −2 ( 2, −2 ) ( 2, −2 )
−3 −3 3 ( −3, 3 ) ( −3, 3 )

A graph of a line passing through three points with coordinates negative three, three; zero, zero; and two, negative two.

Notice that the x− x− and y-intercepts y-intercepts are the same point. Thus the intercept method does not provide enough information to construct this graph.

When an equation is given in the general form ax+by=c ax+by=c , usually the most efficient approach to constructing the graph is to use the intercept method, when it works.

Practice Set B

Graph the following equations.

Exercise 2

x−5y=5 x−5y=5
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates zero, negative one and five, zero.

Exercise 3

x+2y=6 x+2y=6
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates zero, three and two, two.

Exercise 4

2x+y=1 2x+y=1
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates zero, one and one, negative one.

Slanted, Horizontal, and Vertical Lines

In all the graphs we have observed so far, the lines have been slanted. This will always be the case when both variables appear in the equation. If only one variable appears in the equation, then the line will be either vertical or horizontal. To see why, let’s consider a specific case:

Using the general form of a line, ax+by=c ax+by=c , we can produce an equation with exactly one variable by choosing a=0 a=0 , b=5 b=5 , and c=15 c=15 . The equation ax+by=c ax+by=c then becomes

0x+5y=15 0x+5y=15

Since 0⋅( any number )=0 0⋅( any number )=0 , the term 0x 0x is 0 0 for any number that is chosen for x x .

Thus,

0x+5y=15 0x+5y=15

becomes

0+5y=15 0+5y=15

But, 0 0 is the additive identity and 0+5y=5y 0+5y=5y .

5y=15 5y=15

Then, solving for y y we get

y=3 y=3

This is an equation in which exactly one variable appears.

This means that regardless of which number we choose for x x , the corresponding y-value y-value is 3. Since the y-value y-value is always the same as we move from left-to-right through the x-values x-values , the height of the line above the x-axis x-axis is always the same (in this case, 3 units). This type of line must be horizontal.

An argument similar to the one above will show that if the only variable that appears is x x , we can expect to get a vertical line.

Sample Set C

Example 6

Graph y=4 y=4 .
The only variable appearing is y y . Regardless of which x-value x-value we choose, the y-value y-value is always 4. All points with a y-value y-value of 4 satisfy the equation. Thus we get a horizontal line 4 unit above the x-axis x-axis .

Table 4
x x y y ( x, y ) ( x, y )
−3 −3 4 ( −3, 4 ) ( −3, 4 )
−2 −2 4 ( −2, 4 ) ( −2, 4 )
−1 −1 4 ( −1, 4 ) ( −1, 4 )
0 4 ( 0, 4 ) ( 0, 4 )
14 ( 1, 4 ) ( 1, 4 )
24 ( 2, 4 ) ( 2, 4 )
34 ( 3, 4 ) ( 3, 4 )
44 ( 4, 4 ) ( 4, 4 )

A graph of a line parallel to x-axis passing through a point with coordinates zero, four.

Example 7

Graph x=−2 x=−2 .
The only variable that appears is x x . Regardless of which y-value y-value we choose, the x-value x-value will always be −2 −2 . Thus, we get a vertical line two units to the left of the y-axis y-axis .

Table 5
x x y y ( x,y ) ( x,y )
−2 −2 −4 −4 ( −2, −4 ) ( −2, −4 )
−2 −2 −3 −3 ( −2, −3 ) ( −2, −3 )
−2 −2 −2 −2 ( −2, −2 ) ( −2, −2 )
−2 −2 −1 −1 ( −2, −1 ) ( −2, −1 )
−2 −2 0 ( −2, 0 ) ( −2, 0 )
−2 −2 1 ( −2, 1 ) ( −2, 1 )
−2 −2 2 ( −2, 0 ) ( −2, 0 )
−2 −2 3 ( −2, 3 ) ( −2, 3 )
−2 −2 4 ( −2, 4 ) ( −2, 4 )
  • A graph of a line parallel to y-axis and passing through a point with coordinates negative two, zero.

Practice Set C

Exercise 5

Graph y=2 y=2 .
 An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line parallel to x axis passing through a point with coordinates zero, two.

Exercise 6

Graph x=−4 x=−4 .
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line parallel to y axis and passing through a point with coordinates negative four, zero.

Summarizing our results we can make the following observations:

  1. When a linear equation in two variables is written in the form ax+by=c ax+by=c , we say it is written in general form.
  2. To graph an equation in general form it is sometimes convenient to use the intercept method.
  3. A linear equation in which both variables appear will graph as a slanted line.
  4. A linear equation in which only one variable appears will graph as either a vertical or horizontal line.

     x=a x=a graphs as a vertical line passing through a a on the x-axis x-axis .
     y=b y=b graphs as a horizontal line passing through b b on the y-axis y-axis .

Exercises

For the following problems, graph the equations.

Exercise 7

−3x+y=−1 −3x+y=−1
An xy-plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through three points with coordinates negative one, negative four; zero, negative one; and one, two.

Exercise 8

3x−2y=6 3x−2y=6
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 9

−2x+y=4 −2x+y=4
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates negative two, zero and zero, four.

Exercise 10

x−3y=5 x−3y=5
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 11

2x−3y=6 2x−3y=6
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through three points with coordinates negative one, negative eight over three; zero, negative two; and three, zero.

Exercise 12

2x+5y=10 2x+5y=10
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 13

3( x−y )=9 3( x−y )=9
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates three, zero and zero, negative three.

Exercise 14

−2x+3y=−12 −2x+3y=−12
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 15

y+x=1 y+x=1
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through three points with coordinates one, zero; zero, one; and three, negative two.

Exercise 16

4y−x−12=0 4y−x−12=0
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 17

2x−y+4=0 2x−y+4=0
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates negative two, zero and zero, four.

Exercise 18

−2x+5y=0 −2x+5y=0
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 19

y−5x+4=0 y−5x+4=0
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through three points with coordinates zero, negative four; four over five, zero; and two, six.

Exercise 20

0x+y=3 0x+y=3
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 21

0x+2y=2 0x+2y=2
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line parallel to x-axis and passing through two points with coordinates negative three, one and three, one.

Exercise 22

0x+ 1 4 y=1 0x+ 1 4 y=1
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 23

4x+0y=16 4x+0y=16
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line parallel to y-axis and passing through three points with coordinates four, zero; four, two; and four, four.

Exercise 24

1 2 x+0y=−1 1 2 x+0y=−1
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 25

2 3 x+0y=1 2 3 x+0y=1
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

x= 3 2 x= 3 2

A graph of a line parallel to y-axis in an xy plane. The line is labeled as ' x equals three over two' and it crosses the x-axis at x equals three over two.

Exercise 26

y=3 y=3
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 27

y=−2 y=−2
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

y=−2 y=−2

A graph of a line parallel to x-axis in an xy plane. The line is labeled as ' y equals negative two'. The line crosses the y-axis at y equals negative two.

Exercise 28

−4y=20 −4y=20
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 29

x=−4 x=−4
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line parallel to y-axis in an xy plane. The line is labeled as ' x equals negative four'. The line crosses the x-axis at x equals negative four.

Exercise 30

−3x=−9 −3x=−9
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 31

−x+4=0 −x+4=0
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line parallel to y-axis in an xy plane. The line is labeled as ' x equals four'. The line crosses the x-axis at x equals four.

Exercise 32

Construct the graph of all the points that have coordinates ( a, a ) ( a, a ) , that is, for each point, the x− x− and y-values y-values are the same.
 An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Use a calculator Calculator Problems

Exercise 33

2.53x+4.77y=8.45 2.53x+4.77y=8.45
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates three point three four, zero and zero, one point seven seven.

Exercise 34

1.96x+2.05y=6.55 1.96x+2.05y=6.55
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercise 35

4.1x−6.6y=15.5 4.1x−6.6y=15.5
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Solution

A graph of a line passing through two points with coordinates three point seven eight, zero and zero, negative two point three five.

Exercise 36

626.01x−506.73y=2443.50 626.01x−506.73y=2443.50
An xy coordinate plane with gridlines, labeled negative five and five on the both axes.

Exercises for Review

Exercise 37

((Reference)) Name the property of real numbers that makes 4+x=x+4 4+x=x+4 a true statement.

Solution

commutative property of addition

Exercise 38

((Reference)) Supply the missing word. The absolute value of a number a a , denoted | a | | a | , is the
from a a to 0 0 on the number line.

Exercise 39

((Reference)) Find the product ( 3x+2 )( x−7 ) ( 3x+2 )( x−7 ) .

Solution

3 x 2 −19x−14 3 x 2 −19x−14

Exercise 40

((Reference)) Solve the equation 3[ 3( x−2 )+4x ]−24=0 3[ 3( x−2 )+4x ]−24=0 .

Exercise 41

((Reference)) Supply the missing word. The coordinate axes divide the plane into four equal regions called
.

Solution

quadrants

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Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks

Module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks