8.9 Use direct and inverse variation

When two quantities are related by a proportion, we say they are proportional to each other. Another way to express this relation is to talk about the variation of the two quantities. We will discuss direct variation and inverse variation in this section.

Solve direct variation problems

Lindsay gets paid $15 per hour at her job. If we let s be her salary and h be the number of hours she has worked, we could model this situation with the equation

Lindsay’s salary is the product of a constant, 15, and the number of hours she works. We say that Lindsay’s salary varies directly with the number of hours she works. Two variables vary directly if one is the product of a constant and the other.

In applications using direct variation, generally we will know values of one pair of the variables and will be asked to find the equation that relates x and y . Then we can use that equation to find values of y for other values of x .

How to solve direct variation problems

If y varies directly with x and $y=20$ when $x=8$ , find the equation that relates x and y .

Solution

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We’ll list the steps below.

Now we’ll solve a few applications of direct variation.

When Raoul runs on the treadmill at the gym, the number of calories, c , he burns varies directly with the number of minutes, m , he uses the treadmill. He burned 315 calories when he used the treadmill for 18 minutes.

1. ⓐ Write the equation that relates c and m .
2. ⓑ How many calories would he burn if he ran on the treadmill for 25 minutes?

Solution

ⓐ

	The number of calories, $c$ , varies directly with the number of minutes, $m$ , on the treadmill, and $c=315$ when $m=18$ .
Write the formula for direct variation.
We will use $c$ in place of $y$ and $m$ in place of $x$ .
Substitute the given values for the variables.
Solve for the constant of variation.
Write the equation that relates $c$ and $m$ .
Substitute in the constant of variation.

ⓑ

	Find $c$ when $m=25$ .
Write the equation that relates $c$ and $m$ .
Substitute the given value for $m$ .
Simplify.
	Raoul would burn 437.5 calories if he used the treadmill for 25 minutes.

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