Direct variation means the same as direct proportion. There is direct variation (or direct proportion) between two variables when one variable is a simple multiple of the other. That is, their ratio is a constant. • x varies as y
• x varies directly as y
• x is proportional to y
These three all mean the same and they are written in symbols as follows.
x ∝ y. The ‘∝’ sign can always be replaced by ‘=k’ where k is a constant equal to the gradient of the graph and it is called the constant of proportionality or constant of variation.