SECTION 3.2 The Slope of a Line 155
Interpreting Slope as Average Rate of ChangeDuring the year 2000, the average person in the United States spent 812 hr watching
broadcast TV. In 2005, the average number of hours per person spent watching broad-
cast TV was 679. Find the average rate of change in number of hours per year.
(Source:Veronis Suhler Stevenson.)
To use the slope formula, we let one ordered pair be and the other
be
The graph in FIGURE 24confirms that the line through the ordered pairs falls from left
to right and therefore has negative slope. Thus, the average time per person spent
watching broadcast TV decreasedby about 27 hr per year from 2000 to 2005.
average rate of change=
679 - 812
2005 - 2000
=
- 133
5
= -26.6
1 2005, 679 2.
1 2000, 812 2
EXAMPLE 10
2000 20051000
900700
600
500
xyYearHours (per person)Watching Broadcast TV(2000, 812)(2005, 679)0800FIGURE 24 NOW TRYNOW TRY
EXERCISE 10
In 2000, sales of digital cam-
corders in the United States
totaled $2838 million. In
2008, sales totaled $1885
million. Find the average rate
of change in sales of digital
camcorders per year, to the
nearest million dollars.
(Source: Consumer Electron-
ics Association.)
Complete solution available
on the Video Resources on DVD
3.2 EXERCISES
Concept Check Answer each question about slope in Exercises 1– 4.
1.A hill rises 30 ft for every horizontal 100 ft.
Which of the following express its slope
(or grade)? (There are several correct choices.)A.0.3 B. C. D.E. F. 30 G.30% H.
2.If a walkway rises 2 ft for every 10 ft on the horizon-
tal, which of the following express its slope (or
grade)? (There are several correct choices.)A.0.2 B. C. D.20%E. 5 F. G. H. - 5
10
2
20
100
1
5
2
10
-
10
3
10
3
30
100
3
1
3
3
10
30 ft100 ft2 ft
10 ftNOW TRY ANSWER
10.-$119million per yr
A negative
slope indicates a
decrease.