# In this Section:

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.