<P> it follows that if y is directly proportional to x, with (nonzero) proportionality constant k, then x is also directly proportional to y, with proportionality constant 1 / k . </P> <P> If y is directly proportional to x, then the graph of y as a function of x is a straight line passing through the origin with the slope of the line equal to the constant of proportionality: it corresponds to linear growth . </P> <P> The concept of inverse proportionality can be contrasted with direct proportionality . Consider two variables said to be "inversely proportional" to each other . If all other variables are held constant, the magnitude or absolute value of one inversely proportional variable decreases if the other variable increases, while their product (the constant of proportionality k) is always the same . </P> <P> Formally, two variables are inversely proportional (also called varying inversely, in inverse variation, in inverse proportion, in reciprocal proportion) if each of the variables is directly proportional to the multiplicative inverse (reciprocal) of the other, or equivalently if their product is a constant . It follows that the variable y is inversely proportional to the variable x if there exists a non-zero constant k such that </P>

A type of variation such that as one variable increases the other decreases