y results. In the figure, these points are plotted and a line is drawn through them.

x

y

0 9

1 12

2 15

3 18

4 21

5 24

6 27

TABLE

A1

Values

for the

Slope

Intercept

Equation

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A • The Use of Mathematics in Principles of Economics 503

FIGURE A1 Slope and the Algebra of Straight Lines This line graph has xon the horizontal axis and y on the vertical

axis. The y-intercept—that is, the point where the line intersects the y-axis—is 9. The slope of the line is 3; that is,

there is a rise of 3 on the vertical axis for every increase of 1 on the horizontal axis. The slope is the same all along a

straight line.

This example illustrates how the b and m terms in an equation for a straight line determine the shape of the

line. The b term is called the y-intercept. The reason for this name is that, if x = 0, then the b term will reveal
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