<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 . As an example, the time taken for a journey is inversely proportional to the speed of travel . </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> <Dl> <Dd> y = (k x) (\ displaystyle y = \ left ((\ frac (k) (x)) \ right)) </Dd> </Dl> <Dd> y = (k x) (\ displaystyle y = \ left ((\ frac (k) (x)) \ right)) </Dd>

What is a constant of proportionality in math