Two quantities x and y are said to vary in inverse proportion, if there exists a relation of the type xy = k between them, k being a constant.
Given, two quantities x and y are said to vary in inverse proportion. Then, x∝1y or y∝1x. In any case, we get xy=constant Hence the given statemnet is true.
When two quantities are related in such a manner that if one increases and the other decreases, then they always vary inversely.
Two quantities x and y are said to vary directly with each other if for some rational number k, xy =k.