when x decreases, y increases. Graphically, a negative slope means that, as the line on the line graph moves

from left to right, the line falls. The altitude-air density relationship, shown in Figure A4 later in this appendix,

has a negative slope. We will learn that price and quantity demanded have a negative relationship; that is,

consumers will purchase less when the price is higher.

A slope of zero means that there is no relationship between x and y. Graphically, the line is flat; that is, zero rise

over the run. Figure A5 of the unemployment rate, shown later in this appendix, illustrates a common pattern

of many line graphs: some segments where the slope is positive, other segments where the slope is negative,

and still other segments where the slope is close to zero.

The slope of a straight line between two points can be calculated in numerical terms. To calculate slope, begin
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