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In this section, we review the concept of variation. The most commonly encountered type of variation is known as direct variation. With direct variation, y varies directly with (as) x, if there exists a constant k such that: y=kx. The variable k is known as the constant of variation or the constant of proportionality. With direct variation, if k > 0, then as x increases, y increases, or as x decreases, y decreases. To solve a typical direct variation problem, we substitute given values for x and y to find the value for k. This value is a constant and does not change. We then plug in for k and the value given for x and get a value for y. We will also encounter direct variation as a power, inverse variation, inverse variation as a power, joint variation, and combined variation.
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