1. § 2.4
Linear Functions and Slope

2. Linear Functions
All equations of the form Ax + By = C are straight lines when graphed, as long as A and B are not both zero.
Such an equation is called the standard form of the equation of the line.
To graph equations of this form, two very important points are used – the intercepts.
Blitzer, Intermediate Algebra, 5e – Slide #2 Section 2.4

3. Using Intercepts to Graph Ax + By = C
x- and y-Intercepts
Using Intercepts to Graph Ax + By = C
1) Find the x-intercept. Let y = 0 and solve for x.
2) Find the y-intercept. Let x = 0 and solve for y.
3) Find a checkpoint, a third ordered-pair solution.
4) Graph the equation by drawing a line through the three points.
Blitzer, Intermediate Algebra, 5e – Slide #3 Section 2.4

4. Graphing Using Intercepts
EXAMPLE
Graph the equation using intercepts.
SOLUTION
1) Find the x-intercept. Let y = 0 and then solve for x.
Replace y with 0
Multiply and simplify
Divide by -2
The x-intercept is -6, so the line passes through (-6,0).
Blitzer, Intermediate Algebra, 5e – Slide #4 Section 2.4

5. Graphing Using Intercepts
CONTINUED
2) Find the y-intercept. Let x = 0 and then solve for y.
Replace x with 0
Multiply and simplify
Divide by 4
The y-intercept is 3, so the line passes through (0,3).
Blitzer, Intermediate Algebra, 5e – Slide #5 Section 2.4

6. Graphing Using Intercepts
CONTINUED
3) Find a checkpoint, a third ordered-pair solution. For our checkpoint, we will let x = 1 (because x = 1 is not the x-intercept) and find the corresponding value for y.
Replace x with 1
Multiply
Add 2 to both sides
Divide by 4 and simplify
The checkpoint is the ordered pair (1,3.5).
Blitzer, Intermediate Algebra, 5e – Slide #6 Section 2.4

7. Graphing Using Intercepts
CONTINUED
4) Graph the equation by drawing a line through the three points. The three points in the figure below lie along the same line. Drawing a line through the three points results in the graph of
(0,3)
(1,3.5)
(-6,0)
Blitzer, Intermediate Algebra, 5e – Slide #7 Section 2.4

8. Slope of a Line
The slope m of the line through the two distinct points
and
is m, where
Note that you may call either point
However, don’t subtract in one order in the numerator and use another order for subtraction in the denominator - or the sign of your slope will be wrong.
Blitzer, Intermediate Algebra, 5e – Slide #8 Section 2.4

9. Slope as Rate of Change
The change in the y values is the vertical change or the “Rise”.
The change in the x values is the horizontal change or the “Run.”
m is commonly used to denote the slope of a line. The absolute value of a line is related to its steepness.
When the slope is positive, the line rises from left to right.
When the slope is negative, the line falls from left to right.
Blitzer, Intermediate Algebra, 5e – Slide #9 Section 2.4

10. Slopes of Lines
EXAMPLE
Find the slope of the line passing through the pair of points. Then explain what the slope means.
(-7,-8) and (4,-2)
SOLUTION
Remember, the slope is obtained as follows:
This means that for every six units that the line (that passes through both points) travels up, it also travels 11 units to the right.
Blitzer, Intermediate Algebra, 5e – Slide #10 Section 2.4

11. Possibilities for a Line’s Slope
Slopes of Lines
Possibilities for a Line’s Slope
Positive Slope
Negative Slope
Zero Slope
Undefined Slope
Blitzer, Intermediate Algebra, 5e – Slide #11 Section 2.4

12. Slope Intercept Form
In the form of the equation of the line where the equation is written as y = mx + b, it is true that 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 you can easily see both of these, the slope and the y intercept, in the actual equation.
Blitzer, Intermediate Algebra, 5e – Slide #12 Section 2.4

13. Slope-Intercept Form of the Equation of a Line
Slopes of Lines
Slope-Intercept Form of the Equation of a Line
y = mx + b with slope m, and y-intercept b
Examples
y = .968x – m = .968; b = -17.3
f (x) = 3-5x m = -5; b = 3
Blitzer, Intermediate Algebra, 5e – Slide #13 Section 2.4

14. Slopes of Lines
EXAMPLE
Give the slope and y-intercept for the line whose equation is:
3x - 5y = 7
SOLUTION
Hint: You should solve for y so as to have the equation in slope intercept form.
3x - 5y = 7
3x - 3x - 5y = -3x+7
Subtract 3x from both sides
-5y = -3x+7
Simplify
Divide both sides by -5
Simplify
Therefore, the slope is 3/5 and the y-intercept is -7/5.
Blitzer, Intermediate Algebra, 5e – Slide #14 Section 2.4

15. Graphing y = mx + b Using the Slope and y-Intercept
Slopes of Lines
Graphing y = mx + b Using the Slope and y-Intercept
1) 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 containing the y-intercept, 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
Blitzer, Intermediate Algebra, 5e – Slide #15 Section 2.4

16. Graphing Lines Graph the line whose equation is f (x) = 2x + 3.
EXAMPLE
Graph the line whose equation is f (x) = 2x + 3.
SOLUTION
The equation f (x) = 2x + 3 is in the form y = mx + b.
The slope is 2 and the y-intercept is 3.
Now that we have identified the slope and the y-intercept, we use the three steps in the box to graph the equation.
Blitzer, Intermediate Algebra, 5e – Slide #16 Section 2.4

17. Graphing Lines
CONTINUED
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.
(0,3)
Blitzer, Intermediate Algebra, 5e – Slide #17 Section 2.4

18. Graphing Lines
CONTINUED
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.
(1,5)
(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,-1), shown on the graph.
Blitzer, Intermediate Algebra, 5e – Slide #18 Section 2.4

19. Graphing Lines
CONTINUED
3) Use a straightedge to draw a line through the two points. The graph of y = 2x + 3 is show below.
(1,5)
(0,3)
Blitzer, Intermediate Algebra, 5e – Slide #19 Section 2.4

20. Horizontal and Vertical Lines
Equation of a Horizontal Line
(0,b)
A horizontal line is given by an equation of the form y = b where b is the y-intercept.
Equation of a Vertical Line
(a,0)
A vertical line is given by an equation of the form x = a where a is the x-intercept.
Blitzer, Intermediate Algebra, 5e – Slide #20 Section 2.4

21. Horizontal and Vertical Lines
EXAMPLE
Graph the linear equation: y = -4.
SOLUTION
All ordered pairs that are solutions of y = -4 have a value of y that is always -4. Any value can be used for x. Let’s select three of the possible values for x: -3, 1, 6: x
y = -4
(x,y) -3 -4
(-3,-4) 1
(1,-4) 6
(6,-4)
(1,-4)
(-3,-4)
(6,-4)
Upon plotting the three resultant points and connecting the points with a line, the graph to the right is the solution.
Blitzer, Intermediate Algebra, 5e – Slide #21 Section 2.4

22. Horizontal and Vertical Lines
EXAMPLE
Graph the linear equation: x = 5.
SOLUTION
All ordered pairs that are solutions of x = 5 have a value of x that is always 5. Any value can be used for y. Let’s select three of the possible values for y: -3, 1, 5:
(5,5)
x = 5 y
(x,y) 5 -3
(5,-3) 1
(5,1)
(5,5)
(5,1)
(5,-3)
Upon plotting the three resultant points and connecting the points with a line, the graph to the right is the solution.
Blitzer, Intermediate Algebra, 5e – Slide #22 Section 2.4

23. Slope as Rate of Change
EXAMPLE
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 in dollars of a retail drug prescription in the United States, y, x years after 1991.
SOLUTION
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.
Blitzer, Intermediate Algebra, 5e – Slide #23 Section 2.4

24. Linear Modeling
EXAMPLE
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.
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.
SOLUTION
I(x) = 21x + 84, where I(x) is the number of U.S. Internet users (in millions) x years after We predict in 2010, there will be I(12) = 21(12) + 84 = 336 million U.S. Internet users.
Blitzer, Intermediate Algebra, 5e – Slide #24 Section 2.4