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Copyright © 2013, 2009, 2006 Pearson Education, Inc. 1Copyright © 2013, 2009, 2006 Pearson Education, Inc. 1
Section 3.4
The Slope-Intercept Formof the Equation
of a Line
Copyright © 2013, 2009, 2006 Pearson Education, Inc. 1
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Objective #1 Find a line’s slope and y-intercept
from its equation.
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Slope-Intercept Form
In the form of the equation of the line where the equation is written as y = mx + b, if x = 0, then y = b.
Therefore, b is the y-intercept for the equation of the line. Furthermore, the x coefficient m is the slope of the line.
This form of the equation is called the “Slope- Intercept Form” since we can easily see both of these, the slope and the y-intercept, in the actual equation.
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Slopes of Lines
Slope-Intercept Form of the Equation of a Line
y = mx + b with slope m, and y-intercept b
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Find the slope and y-intercept of the line y = 2x – 7
The slope is 2.
Slope-Intercept Form
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Slopes of Lines
EXAMPLEEXAMPLE
SOLUTIONSOLUTION
Give the slope and y-intercept for the line whose equation is:
5 3 7
5 5
y x
5
7
5
3 xy
Subtract 3x from both sides
Simplify
Simplify
Hint: You should solve for y so as to have the equation in slope intercept form.
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EXAMPLE EXAMPLE Find the slope and y-intercept of the line 2x + 3y = 9
33
23
9
3
23
92
3
3
923
932
xy
xy
xy
xy
yx
Slope – Intercept Form
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Objective #1: Examples
1a. Find the slope and the y-intercept of the line: 2
43
y x
The slope is the x-coefficient, which is 2
.3
m
The y-intercept is the constant term, which is 4.
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Objective #1: Examples
1a. Find the slope and the y-intercept of the line: 2
43
y x
The slope is the x-coefficient, which is 2
.3
m
The y-intercept is the constant term, which is 4.
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Objective #1: Examples
1b. Find the slope and the y-intercept of the line: 7 6x y
First, solve the equation for y. 7 6 7 6x y y x
The slope is the x-coefficient, which is 7.m
The y-intercept is the constant term, which is 6.
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Objective #1: Examples
1b. Find the slope and the y-intercept of the line: 7 6x y
First, solve the equation for y. 7 6 7 6x y y x
The slope is the x-coefficient, which is 7.m
The y-intercept is the constant term, which is 6.
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Objective #2 Graph lines in slope-intercept form.
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Slopes of Lines
Graphing y = mx + b Using the Slope and y-Intercept1) Plot the point containing the y-intercept on the y-axis. This is the point
(0,b).
2) Obtain a second point using the slope, m. Write m as a fraction, and use rise over run, starting at the point on the y-axis, to plot this point.
3) Use a straightedge to draw a line through the two points. Draw arrowheads at the ends of the line to show that the line continues indefinitely in both directions
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Graphing Lines
EXAMPLEEXAMPLE
SOLUTIONSOLUTION
Graph the line whose equation is y = 2x + 3.
The equation y = 2x + 3 is in the form y = mx + b.
Now that we have identified the slope and the y-intercept, we use the three steps in the box to graph the equation.
The slope is 2 and the y-intercept is 3.
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Graphing Lines
1) Plot the point containing the y-intercept on the y-axis. The y-intercept is 3. We plot the point (0,3), shown below.
CONTINUEDCONTINUED
(0,3)
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Graphing Lines
2) Obtain a second point using the slope, m. Write m as a fraction, and use rise over run, starting at the point containing the y-intercept, to plot this point. We express the slope, 2, as a fraction.
CONTINUEDCONTINUED
(0,3)We plot the second point on the line by starting at (0, 4), the first point. Based on the slope, we move 2 units up (the rise) and 1 unit to the right (the run). This puts us at a second point on the line, (1, 5), shown on the graph.
(1,5)Run
Rise
1
2m
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Graphing Lines
3) Use a straightedge to draw a line through the two points. The graph of y = 2x + 3 is show below.
CONTINUEDCONTINUED
(0,3)(1,5)
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Objective #2: Examples
2a. Graph: 3 2y x
The y-intercept is –2, so plot the point (0, 2).
The slope is 3
3 or .1
m m
Find another point by going up 3 units and to the right 1 unit.
Use a straightedge to draw a line through the two points.
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Objective #2: Examples
2a. Graph: 3 2y x
The y-intercept is –2, so plot the point (0, 2).
The slope is 3
3 or .1
m m
Find another point by going up 3 units and to the right 1 unit.
Use a straightedge to draw a line through the two points.
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Objective #2: Examples
2b. Graph: 3
15
y x
The y-intercept is 1, so plot the point (0,1).
The slope is 3
.5
m
Find another point by going up 3 units and to the right 5 units.
Use a straightedge to draw a line through the two points.
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Objective #2: Examples
2b. Graph: 3
15
y x
The y-intercept is 1, so plot the point (0,1).
The slope is 3
.5
m
Find another point by going up 3 units and to the right 5 units.
Use a straightedge to draw a line through the two points.
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Objective #3 Use slope and y-intercept to graph Ax + By = C.
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Graph using the Slope and y-Intercept
Earlier in this chapter, we considered linear equations of the form . We used x- and y-intercepts, as well as checkpoints, to graph these equations. It is also possible to obtain graphs by using the slope and y-intercept. To do this, begin by solving for y. This will put the equation in slope-intercept form. Then use the three-step procedure to graph the equation.
Ax By C
Ax By C
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3. Graph 3 4 0x y by using slope and y-intercept.
Solve for y: 3 4 0
4 3
34
x y
y x
y x
The y-intercept is 0, so plot the point (0,0).
The slope is 3
.4
m
Objective #3: Example
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3. Graph 3 4 0x y by using slope and y-intercept.
Solve for y: 3 4 0
4 3
34
x y
y x
y x
The y-intercept is 0, so plot the point (0,0).
The slope is 3
.4
m
Objective #3: Example
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Objective #3: Example
Find another point by going down 3 units and to the right 4 units. Use a straightedge to draw a line through the two points.
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Objective #4 Use slope and y-intercept to model data.
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Slope as Rate of Change
EXAMPLEEXAMPLE
A linear function that models data is described. Find the slope of each model. Then describe what this means in terms of the rate of change of the dependent variable y per unit change in the independent variable x.
The linear function y = 2x + 24 models the average cost, y in dollars of a retail drug prescription in the United States, x years after 1991.
SOLUTIONSOLUTION
The slope of the linear model is 2. This means that every year (since 1991) the average cost in dollars of a retail drug prescription in the U.S. has increased approximately $2.
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Linear Modeling
EXAMPLEEXAMPLE
Find a linear function, in slope-intercept form or a constant function that models the given description. Describe what each variable in your model represents. Then use the model to make a prediction of the number of U.S. Internet users in 2010.
SOLUTIONSOLUTION
In 1998, there were 84 million Internet users in the United States and this number has increased at a rate of 21 million users per year since then.
I(x) = 21x + 84, where I(x) is the number of U.S. Internet users (in millions) x years after 1998. We predict in 2010, there will be I(12) = 21(12) + 84 = 336 million U.S. Internet users.
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4. The figure shows the percentage of the U.S. population who had graduated from high school and from college in 1960 and 2010.
Objective #4: Examples
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Objective #4: Examples
4a. Use the two points for college in the figure to find an equation in the form y mx b that models the percentage of college graduates in the U.S. population, y, x years after 1960.
The y-intercept is 8 and the slope is The equation is 0.32 8.y x
Change in 24 8 160.32
Change in 50 0 50y
mx
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Objective #4: Examples
4a. Use the two points for college in the figure to find an equation in the form y mx b that models the percentage of college graduates in the U.S. population, y, x years after 1960.
The y-intercept is 8 and the slope is The equation is 0.32 8.y x
Change in 24 8 160.32
Change in 50 0 50y
mx
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Objective #4: Examples
4b. Use the model from part (a) to project the percentage of college graduates in 2020.
0.32 8
0.32(60) 8
27.2
y x
The model projects that 27.2% of the U.S. population will be college graduates in 2020.
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Objective #4: Examples
4b. Use the model from part (a) to project the percentage of college graduates in 2020.
0.32 8
0.32(60) 8
27.2
y x
The model projects that 27.2% of the U.S. population will be college graduates in 2020.