Beginning Algebra, 11th Edition

Interpreting a Line Graph

The line graph in FIGURE 1shows average prices of a gallon of regular unleaded gaso-

line in the United States for the years 2001 through 2008.

EXAMPLE 1

176 CHAPTER 3 Linear Equations and Inequalities in Two Variables; Functions

OBJECTIVES OBJECTIVE 1 Interpret graphs.A line graph is used to show changes or trends

in data over time. To form a line graph,we connect a series of points representing

data with line segments.

Linear Equations in Two Variables; The Rectangular Coordinate System

3.1

1 Interpret graphs. 2 Write a solution as an ordered pair. 3 Decide whether a given ordered pair is a solution of a given equation. 4 Complete ordered pairs for a given equation. 5 Complete a table of values. 6 Plot ordered pairs.

NOW TRY
EXERCISE 1
Refer to the line graph in
FIGURE 1.

(a)Estimate the average price
of a gallon of gasoline in
2006.

(b)About how much did the
average price of a gallon
of gasoline increase from
2006 to 2008?

0

1.20

1.40

1.60

1.80

2.00

2.20

2.40

2.60

2.80

3.00

3.20

3.40

Price (in dollars per gallon)

Year

'01 '02 '03 '04 '05 '06 '07 '08

Source: U.S. Department of Energy.

Average U.S. Gasoline Prices

FIGURE 1

(a)Between which years did the average price of a gallon of gasoline decrease?

The line between 2001 and 2002 falls, so the average price of a gallon of gaso-

line decreased from 2001 to 2002.

(b)What was the general trend in the average price of a gallon of gasoline from 2002

through 2008?

The line graph rises from 2002 to 2008, so the average price of a gallon of gaso-

line increased over those years.

(c) Estimate the average price of a gallon of gasoline in 2002 and 2008. About how

much did the price increase between 2002 and 2008?

Move up from 2002 on the horizontal scale to the point plotted for 2002. Look-

ing across at the vertical scale, this point is about three-fourths of the way between

the lines on the vertical scale for $1.20 and $1.40. Halfway between the lines for

$1.20 and $1.40 would be $1.30. So, it cost about $1.35 for a gallon of gasoline in

2002.

Similarly, move up from 2008 on the horizontal scale to the point plotted for

2008. Then move across to the vertical scale. The price for a gallon of gasoline in

2008 was about $3.25.

Between 2002 and 2008, the average price of a gallon of gasoline increased by

about

$3.25-$1.35=$1.90. NOW TRY

NOW TRY ANSWERS

(a)about $2.60
(b)about $0.65

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