Question

Threethings determine if a relationship is proportional

Answer 1

If x is one variable in the relationship and y is the other, then xy=constant for all values of x and y (that's inverse proportionality); or y=ax (where a is a constant) for direct proportionality, so that y=0 when x=0. So we could say:

1. One variable changes as the other changes: one increases as the other increases in a linear fashion (a graph would be a straight line) – direct proportionality;
2. If directly proportional, the graph is a straight line passing through the origin;
3. One variable changes as the other changes: one increases as the other decreases (non-linear) – inverse proportionality, and their product is constant.