In direct variation, one variable depends only on one other variable and depends on it in a linear fashion. As one quantity increases, the other quantity will also increase at a constant rate. As one quantity decreases, the other quantity decreases at a constant rate. Because the value of d depends on one variable t in formula I and varies at a constant rate of fifty, formula I shows direct variation. Direct variation is represented by the formula y = kx.

In formula II, while the output only depends on the one variable t, it does not depend on it in a linear fashion since t is raised to the second power, so it is not direct variation. In formula III, d depends on two other variables r and t, so it is not direct variation.