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The figure shows a straight horizontal line drawn through three points on the x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. Dots mark off the three points which are labeled by their ordered pairs (0, 4), (2, 4), and (4, 4). A straight horizontal line goes through all three points. The line has arrows on both ends pointing to the outside of the figure. The line is labeled with the equation y equals 4.

Horizontal line

A horizontal line    is the graph of an equation of the form y = b .

The line passes through the y -axis at ( 0 , b ) .

 

Graph the equation y = −1 .

Solution

The equation y = −1 has only one variable, y . The value of y is constant. All the ordered pairs in [link] have the same y -coordinate. The graph is a horizontal line passing through the y -axis at −1 , as shown in [link] .

y = −1
x y ( x , y )
0 −1 ( 0 , −1 )
3 −1 ( 3 , −1 )
−3 −1 ( −3 , −1 )
The figure shows a straight horizontal line drawn through three points on the x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. Dots mark off the three points which are labeled by their ordered pairs (negative 3, negative 1), (0, negative 1), and (3, negative 1). A straight horizontal line goes through all three points. The line has arrows on both ends pointing to the outside of the figure. The line is labeled with the equation y equals negative 1.
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Graph the equation y = −4 .

The figure shows a straight horizontal line drawn on the x y-coordinate plane. The x-axis of the plane runs from negative 12 to 12. The y-axis of the plane runs from negative 12 to 12. The straight line goes through the points (negative 4, negative 4), (0, negative 4), (4, negative 4), and all other points with second coordinate negative 4. The line has arrows on both ends pointing to the outside of the figure.

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Graph the equation y = 3 .

The figure shows a straight horizontal line drawn on the x y-coordinate plane. The x-axis of the plane runs from negative 12 to 12. The y-axis of the plane runs from negative 12 to 12. The straight line goes through the points (negative 4, 3), (0, 3), (4, 3), and all other points with second coordinate 3. The line has arrows on both ends pointing to the outside of the figure.

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The equations for vertical and horizontal lines look very similar to equations like y = 4 x . What is the difference between the equations y = 4 x and y = 4 ?

The equation y = 4 x has both x and y . The value of y depends on the value of x . The y -coordinate changes according to the value of x . The equation y = 4 has only one variable. The value of y is constant. The y -coordinate is always 4. It does not depend on the value of x . See [link] .

y = 4 x y = 4
x y ( x , y ) x y ( x , y )
0 0 ( 0 , 0 ) 0 4 ( 0 , 4 )
1 4 ( 1 , 4 ) 1 4 ( 1 , 4 )
2 8 ( 2 , 8 ) 2 4 ( 2 , 4 )
The figure shows a two straight lines drawn on the same x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. One line is a straight horizontal line labeled with the equation y equals 4. The other line is a slanted line labeled with the equation y equals 4x.

Notice, in [link] , the equation y = 4 x gives a slanted line, while y = 4 gives a horizontal line.

Graph y = −3 x and y = −3 in the same rectangular coordinate system.

Solution

Notice that the first equation has the variable x , while the second does not. See [link] . The two graphs are shown in [link] .

y = −3 x y = −3
x y ( x , y ) x y ( x , y )
0 0 ( 0 , 0 ) 0 −3 ( 0 , −3 )
1 −3 ( 1 , −3 ) 1 −3 ( 1 , −3 )
2 −6 ( 2 , −6 ) 2 −3 ( 2 , −3 )
The figure shows a two straight lines drawn on the same x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. One line is a straight horizontal line labeled with the equation y equals negative 3. The other line is a slanted line labeled with the equation y equals negative 3x.
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Graph y = −4 x and y = −4 in the same rectangular coordinate system.

The figure shows a two straight lines drawn on the same x y-coordinate plane. The x-axis of the plane runs from negative 12 to 12. The y-axis of the plane runs from negative 12 to 12. One line is a straight horizontal line going through the points (negative 4, negative 4), (0, negative 4), (4, negative 4), and all other points with second coordinate negative 4. The other line is a slanted line going through the points (negative 2, 8), (negative 1, 4), (0, 0), (1, negative 4), and (2, negative 8).

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Graph y = 3 and y = 3 x in the same rectangular coordinate system.

The figure shows a two straight lines drawn on the same x y-coordinate plane. The x-axis of the plane runs from negative 12 to 12. The y-axis of the plane runs from negative 12 to 12. One line is a straight horizontal line going through the points (negative 4, 3) (0, 3), (4, 3), and all other points with second coordinate 3. The other line is a slanted line going through the points (negative 2, negative 6), (negative 1, negative 3), (0, 0), (1, 3), and (2, 6).

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Key concepts

  • Graph a Linear Equation by Plotting Points
    1. Find three points whose coordinates are solutions to the equation. Organize them in a table.
    2. Plot the points in a rectangular coordinate system. Check that the points line up. If they do not, carefully check your work!
    3. Draw the line through the three points. Extend the line to fill the grid and put arrows on both ends of the line.

Practice makes perfect

Recognize the Relationship Between the Solutions of an Equation and its Graph

In the following exercises, for each ordered pair, decide:

Is the ordered pair a solution to the equation?  Is the point on the line?

y = x + 2

( 0 , 2 )
( 1 , 2 )
( −1 , 1 )
( −3 , −1 )

The figure shows a straight line drawn on the x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. The straight line goes through the points (negative 6, negative 4), (negative 5, negative 3), (negative 4, negative 2), (negative 3, negative 1), (negative 2, 0), (negative 1, 1), (0, 2), (1, 3), (2, 4), (3, 5), (4, 6), and (5, 7).

yes; no  no; no  yes; yes  yes; yes

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y = x 4

( 0 , −4 )
( 3 , −1 )
( 2 , 2 )
( 1 , −5 )

The figure shows a straight line drawn on the x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. The straight line goes through the points (negative 3, negative 7), (negative 2, negative 6), (negative 1, negative 5), (0, negative 4), (1, negative 3), (2, negative 2), (3, negative 1), (4, 0), (5, 1), (6, 2), and (7, 3).
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y = 1 2 x 3

( 0 , −3 )
( 2 , −2 )
( −2 , −4 )
( 4 , 1 )

The figure shows a straight line drawn on the x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. The straight line goes through the points (negative 6, negative 6), (negative 4, negative 5), (negative 2, negative 4), (0, negative 3), (2, negative 2), (4, negative 1), and (6, 0).

yes; yes  yes; yes  yes; yes  no; no

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y = 1 3 x + 2

( 0 , 2 )
( 3 , 3 )
( −3 , 2 )
( −6 , 0 )

The figure shows a straight line drawn on the x y-coordinate plane. The x-axis of the plane runs from negative 7 to 7. The y-axis of the plane runs from negative 7 to 7. The straight line goes through the points (negative 6, 0), (negative 3, 1), (0, 2), (3, 3), and (6, 4).
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Graph a Linear Equation by Plotting Points

In the following exercises, graph by plotting points.

Graph Vertical and Horizontal Lines

In the following exercises, graph each equation.

In the following exercises, graph each pair of equations in the same rectangular coordinate system.

y = 1 2 x and y = 1 2

The figure shows a two straight lines drawn on the same x y-coordinate plane. The x-axis of the plane runs from negative 12 to 12. The y-axis of the plane runs from negative 12 to 12. One line is a straight horizontal line going through the points (negative 4, negative one half) (0, negative one half), (4, negative one half), and all other points with second coordinate negative one half. The other line is a slanted line going through the points (negative 10, 5), (negative 8, 4), (negative 6, 3), (negative 4, 2), (negative 2, 1), (0, 0), (1, negative 2), (2, negative 4), (3, negative 6), (4, negative 8), and (5, negative 10).

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y = 1 3 x and y = 1 3

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Mixed practice

In the following exercises, graph each equation.

Everyday math

Motor home cost. The Robinsons rented a motor home for one week to go on vacation. It cost them $594 plus $0.32 per mile to rent the motor home, so the linear equation y = 594 + 0.32 x gives the cost, y , for driving x miles. Calculate the rental cost for driving 400, 800, and 1200 miles, and then graph the line.

$722, $850, $978
The figure shows a straight line drawn on the x y-coordinate plane. The x-axis of the plane runs from 0 to 1200 in increments of 100. The y-axis of the plane runs from 0 to 1000 in increments of 100. The straight line starts at the point (0, 594) and goes through the points (400, 722), (800, 850), and (1200, 978). The right end of the line has an arrow pointing up and to the right.

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Weekly earnings. At the art gallery where he works, Salvador gets paid $200 per week plus 15% of the sales he makes, so the equation y = 200 + 0.15 x gives the amount, y , he earns for selling x dollars of artwork. Calculate the amount Salvador earns for selling $900, $1600, and $2000, and then graph the line.

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Writing exercises

Explain how you would choose three x - values to make a table to graph the line y = 1 5 x 2 .

Answers will vary.

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What is the difference between the equations of a vertical and a horizontal line?

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Self check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This table has 4 rows and 4 columns. The first row is a header row and it labels each column. The first column header is “I can…”, the second is “Confidently”, the third is “With some help”, and the fourth is “No, I don’t get it”. Under the first column are the phrases “…recognize the relation between the solutions of an equation and its graph.”, “…graph a linear equation by plotting points.”, and “…graph vertical and horizontal lines.”. The other columns are left blank so that the learner may indicate their mastery level for each topic.

After reviewing this checklist, what will you do to become confident for all goals?

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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