(A) Determining whether a quantity varies directly as another quantity
1. If a quantity y varies directly as a quantity x, the
(a) y increases when x increases
(b) y decreases when x decreases
2. A quantity y varies directly as a quantity x if and only if
where k is called the constant of variation.
3. y varies directly as x is written as
.
4. When , the graph of y against x is a straight line passing through the origin.
4. When , the graph of y against x is a straight line passing through the origin.
(B) Expressing a direct variation in the form of an equation involving two variables
Example 1