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This chapter covers principles of linear equations. After completing this chapter students should be able to: graph a linear equation; find the slope of a line; determine an equation of a line; solve linear systems; and complete application problems using linear equations.

Graphing a linear equation

Work the following problems.

Is the point (2, 3) on the line 5x 2y = 4 size 12{5x - 2y=4} {} ?

Yes

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Is the point (1, – 2) on the line 6x y = 4 size 12{6x - y=4} {} ?

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For the line 3x y = 12 size 12{3x - y=12} {} , complete the following ordered pairs.

(2, ) ( , 6)

(0, ) ( , 0)

(2, –6) (6, 6)

(0,–12) (4, 0)

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For the line 4x + 3y = 24 size 12{4x+3y="24"} {} , complete the following ordered pairs.

(3, ) ( , 4)

(0, ) ( , 0)

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Graph y = 2x + 3 size 12{y=2x+3} {}

The line y=2x+3 passing through the points (-1,1) and (0,3) on a Cartesian graph.
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Graph y = 3x + 5 size 12{y= - 3x+5} {}

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Graph y = 4x 3 size 12{y=4x - 3} {}

The line y=4x-3 passing through the points (0,-3) and (2,5) on a Cartesian graph.
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Graph x 2y = 8 size 12{x - 2y=8} {}

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Graph 2x + y = 4 size 12{2x+y=4} {}

The line 2x+y=4 passing through the point (0,4) and (2,0) on a Cartesian graph.
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Graph 2x 3y = 6 size 12{2x - 3y=6} {}

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Graph 2x + 4 = 0 size 12{2x+4=0} {}

A line passing through the point (-2,0) on a Cartesian graph.
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Graph 2y 6 = 0 size 12{2y - 6=0} {}

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Graph the following three equations on the same set of coordinate axes.

y = x + 1   size 12{y=x+1}     {} y = 2x + 1   size 12{y=2x+1}     {} y = x + 1 size 12{y= - x+1} {}
The lines y=-x+1, y=2x+7, and y=x+1 intersecting at the point (0,1) on a Cartesian graph.
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Graph the following three equations on the same set of coordinate axes.

y = 2x + 1   size 12{y=2x+1} {} y = 2x   size 12{y=2x} {} y = 2x 1 size 12{y=2x - 1} {}
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Graph the line using the parametric equations

x = 1 + 2t size 12{x=1+2t} {} , y = 3 + t size 12{y=3+t} {}
A line passing through the points (-5,0), (1,3), and (3,4) on a Cartesian graph.
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Graph the line using the parametric equations

x = 2 3t size 12{x=2 - 3t} {} ,   y = 1 + 2t size 12{y=1+2t} {}
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Slope of a line

Find the slope of the line passing through the following pair of points.

(2, 3) and (5, 9)

m = 2 size 12{m=2} {}
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(– 1, 1) and (1, 3)

m = 1 size 12{m=1} {}
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(6, – 5) and (4, – 1)

m = 2 size 12{m= - 2} {}
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(5, 3) and (– 1, – 4)

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(3, 4) and (3, 7)

m = undefined size 12{m="undefined"} {}
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(– 2, 4) and (– 3, – 2)

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(– 3, – 5) and (– 1, – 7)

m = 1 size 12{m= - 1} {}
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Determine the slope of the line from the given equation of the line.

y = 2x + 1 size 12{y= - 2x+1} {}
m = 2 size 12{m= - 2} {}
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y = 3x 2 size 12{y=3x - 2} {}

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2x y = 6 size 12{2x - y=6} {}
m = 2 size 12{m=2} {}
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x + 3y = 6 size 12{x+3y=6} {}
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3x 4y = 12 size 12{3x - 4y="12"} {}
m = 3 / 4 size 12{m=3/4} {}
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What is the slope of the x-axis? How about the y-axis?

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Graph the line that passes through the given point and has the given slope.

(1, 2) and m = 3 / 4 size 12{m= - 3/4} {}

A line passing through the points (1,2) and (5,-1) on a Cartesian graph.
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(2, –1) and m = 2 / 3 size 12{m=2/3} {}

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(0, 2) and m = 2 size 12{m= - 2} {}

A line passing through the points (0,2) and (1,0) on a Cartesian graph.
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(2, 3) and m = 0 size 12{m=0} {}

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Determining the equation of a line

Write an equation of the line satisfying the following conditions. Write the equation in the form y = mx + b size 12{y= ital "mx"+b} {} .

It passes through the point (3, 10) and has slope = 2 size 12{"slope"=2} {} .

y = 2x + 4 size 12{y=2x+4} {}
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It passes through the point (4,5) and has m = 0 size 12{m=0} {} .

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It passes through (3, 5) and (2, – 1).

y = 6x 13 size 12{y=6x - "13"} {}
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It has slope 3, and its y-intercept equals 2.

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It passes through (5, – 2) and m = 2 / 5 size 12{m=2/5} {} .

y = 2 / 5x 4 size 12{y=2/5x - 4} {}
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It passes through (– 5, – 3) and (10, 0).

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It passes through (4, – 4) and (5, 3).

y = 7x 32 size 12{y=7x - "32"} {}
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It passes through (7, – 2) and its y-intercept is 5.

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It passes through (2, – 5) and its x-intercept is 4.

y = 5 / 2x 10 size 12{y=5/2x - "10"} {}
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Its a horizontal line through the point (2, – 1).

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It passes through (5, – 4) and (1, – 4).

y = 4 size 12{y= - 4} {}
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It is a vertical line through the point (3, – 2).

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It passes through (3, – 4) and (3, 4).

x = 3 size 12{x=3} {}
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It has x -intercept = 3 size 12{x"-intercept"=3} {} and y -intercept = 4 size 12{y"-intercept"=4} {} .

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Write an equation of the line satisfying the following conditions. Write the equation in the form Ax + By = C size 12{ ital "Ax"+ ital "By"=C} {} .

It passes through (3, – 1) and m = 2 size 12{m=2} {} .

2x y = 7 size 12{2x - y=7} {}
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It passes through (– 2, 1) and m = 3 / 2 size 12{m= - 3/2} {} .

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It passes through (– 4, – 2) and m = 3 / 4 size 12{m=3/4} {} .

3x 4y = 4 size 12{3x - 4y= - 4} {}
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Its x-intercept equals 3, and m = 5 / 3 size 12{m= - 5/3} {} .

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It passes through (2, – 3) and (5, 1).

4x 3y = 17 size 12{4x - 3y="17"} {}
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It passes through (1, – 3) and (– 5, 5).

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Applications

In the following application problems, assume a linear relationship holds.

The variable cost to manufacture a product is $25, and the fixed costs are $1200. If x size 12{x} {} represents the number of items manufactured and y size 12{y} {} the cost, write the cost function.

y = 25 x + 1200 size 12{y="25"x+"1200"} {}
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Source:  OpenStax, Applied finite mathematics. OpenStax CNX. Jul 16, 2011 Download for free at http://cnx.org/content/col10613/1.5
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