1. Preview
Warm Up
California Standards
Lesson Presentation

2. Warm Up Part I Add or subtract. 1. 4 + (–6) –2 2. –3 + 5 2 3. –7 – 7
–14
4. 2 – (–1) 3
5. Find the x- and y-intercepts of 2x – 5y = 20.
x-int.: 10; y-int.: –4

3. Warm Up Part II Describe the correlation shown by the scatter plot. 6.7. • • • • • • • • • • • • •
negative
positive

4. California
Standards
Preparation for 8.0 Students understand the concepts of parallel lines and perpendicular lines and how their slopes are related. Students are able to find the equation of a line perpendicular to a given line that passes through a given point. Also covered:

5. Vocabulary
rate of change
rise
run
slope

6. A rate of change is a ratio that compares the amount of change in a dependent variable to the amount of change in an independent variable.
(y)
(x)
For any two points on a nonvertical line, this ratio is constant. The constant rate of change of a nonvertical line is call the slope of the line.

8. Additional Example 1: Finding Slope from a Graph
Find the slope of the line.
Run = –9
Rise = 3
Begin at one point and count vertically to find the rise.
Then count horizontally to the second point to find the run.
Run = 9
Rise = –3

9. Find the slope of the line.
Check It Out! Example 1
Find the slope of the line.
Run = –4
Begin at one point and count vertically to find the rise.
Rise = 2
Then count horizontally to the second point to find the run.
Rise =–2
Run = 4

10. Additional Example 2: Finding Slopes of Horizontal and Vertical Lines
Find the slope of each line. B. A.
The slope is undefined.
The slope is 0.

11. Check It Out! Example 2
Find the slope of each line. a. b.
You cannot divide by 0.
The slope is undefined.
The slope is 0.

12. If you know 2 different points on a line, you can use the slope formula to find the slope of the line.

13. Additional Example 3: Finding Slope by Using the Slope Formula
Find the slope of the line that contains (2, 5) and (8, 1).
Use the slope formula.
Substitute (2, 5) for (x1, y1) and (8, 1) for (x2, y2).
Simplify.

14. The small numbers to the bottom right of the variables are called subscripts. Read x1 as “x sub one” and y2 as “y sub two.”
Reading Math

15. Check It Out! Example 3a
Find the slope of the line that contains (–2, –2) and (7, –2).
Use the slope formula.
Substitute (–2, –2) for (x1, y1) and (7, –2) for (x2, y2).
Simplify.

16. Check It Out! Example 3b
Find the slope of the line that contains (5, –7) and (6, –4).
Use the slope formula.
Substitute (5, –7) for (x1,y1) and (6, –4) for (x2, y2).
Simplify.

17. As shown in the previous examples, slope can be positive, negative, zero, or undefined.

18. Additional Example 4: Describing Slope
Tell whether the slope of each line is positive, negative, zero, or undefined. B. A.
The line rises from left to right.
The line falls from left to right.
The slope is positive.
The slope is negative.

19. Check It Out! Example 4
Tell whether the slope of each line is positive, negative, zero, or undefined. a. b.
The line is vertical.
The line rises from left to right.
The slope is undefined.
The slope is positive.

20. Remember that slope is a rate of change
Remember that slope is a rate of change. In real-world problems, finding the slope can give you information about how a quantity is changing.

21. Additional Example 5: Application
The graph shows the average electricity costs in dollars for operating a refrigerator for several months. Find the slope of the line. Then tell what the slope represents.
Step 1 Use the slope formula.

22. Additional Example 5 Continued
Step 2 Tell what the slope represents.
In this situation, y represents the total cost of electricity and x represents time. So slope represents in units of
A slope of 6 means that the cost of electricity to run the refrigerator is increasing (positive change) at a rate of 6 dollars each month.

23. Check It Out! Example 5
The graph shows the height of a plant over a period of days. Find the slope of the line. Then tell what the slope represents.
Step 1 Use the slope formula.

24. Check It Out! Example 5 Continued
Step 2 Tell what the slope represents.
In this situation, y represents the height of the plant and x represents time. So slope represents
in units of
A slope of means that the height of the plant is increasing (positive change) at a rate of 1 cm every two days.

25. If you know the equation of a line, you can find its slope by using any two ordered-pair solutions. It is often easiest to use the ordered pairs that contain the intercepts.

26. Additional Example 6: Finding Slope from an Equation
Find the slope of the line given by 4x – 2y = 16.
Step 1
Find the x-intercept.
Step 2
Find the y-intercept.
4x – 2y = 16
4x – 2y = 16
4x – 2(0) = 16
Let y = 0.
4(0) – 2y = 16
Let x = 0.
4x = 16
–2y = 16
x = 4
y = –8

27. Additional Example 6 Continued
Step 3 The line contains (4, 0) and (0, –8). Use the slope formula.

28. Check It Out! Example 6
Find the slope of the line given by 2x + 3y = 12.
Step 1
Find the x-intercept.
Step 2
Find the y-intercept.
2x + 3y = 12
2x + 3y = 12
2x + 3(0) = 12
Let y = 0.
4(0) + 3y = 12
Let x = 0.
2x = 12
3y = 12
x = 6
y = 4

29. Check It Out! Example 6
Step 3 The line contains (6, 0) and (0, 4). Use the slope formula.

30. A line’s slope is a measure of its steepness
A line’s slope is a measure of its steepness. Some lines are steeper than others. As the absolute value of the slope increases, the line becomes steeper.

32. Lesson Quiz: Part I
Find the slope of each line. 1. 2.
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33. Lesson Quiz: Part II
3. Find the slope of the line that contains (5, 3) and (–1, 4).
4. Find the slope. Then tell what the slope represents. 50
A slope of 50 means that the speed of the bus is 50 mi/h.
5. Find the slope of the line given by x – 2y = 8.