The general formula for direct variation is k equals y divided by x. In this formula, k is a constant quantity and x and y are variables. K is also called the constant of variation, because it stays the same even as x and y vary.
When designing graphs, the formula is usually expressed as y equals k times x, because it is easier to find points on a graph when solving for one variable and plugging values of the other variable into the formula. This way of expressing the formula also makes it clear that when x increases, y must increase as well. In inverse variation, when one variable increases the other decreases. Inverse variation is considered the opposite of direct variation.