## Precalculus (6th Edition) Blitzer

The condition is such that y varies directly as x and can be modeled by the equation $y\ =\ kx$ where k is called the constant of variation.
The variable y varies directly as x means $y\ \propto \ x$ , which implies that when the variable x increases, y also increases or when x decreases, y also decreases. Using the constant of proportionality k, we have $y\ =\ kx$ Hence, the variable y varies directly as x can be written as $y\ =\ kx$ , where k is called the constant of variation or constant of proportionality.