A proportional relationship is one in which two quantities vary directly with each other. We say the variable y varies directly as x if:
y=kx
for some constant k , called the constant of proportionality .
(Some textbooks describe a proportional relationship by saying that " y varies proportionally with x " or that " y is directly proportional to x .")
This means that as x increases, y increases and as x decreases, y decreases-and that the ratio between them always stays the same.
