In direct proportion, two related quantities, one of which changes and the other changes with it. If the ratio (that is, quotient) of the two numbers corresponding to these two quantities is certain, these two quantities are called direct ratio, and their relationship is called direct ratio. The proportional image is a ray. It can be expressed as x÷y=k (sure) or x=ky.

Inverse ratio of two related quantities, one quantity changes and the other quantity changes. If the product of two numbers corresponding to these two quantities is certain, these two quantities are called inverse proportional quantities, and the relationship between them is called inverse proportional relation, which is expressed as xy=k (certain).