Proportion refers to two related quantities, one of which changes and the other changes. If the ratio (that is, quotient) of the two numbers corresponding to these two quantities is certain, these two quantities are called proportional quantities, and the relationship between them is called proportional relationship.

The area of a circle is πr 2, the radius is R, and their ratio is π r, which is not a fixed value, so it cannot be directly proportional.

For comparison and understanding, for example, the circumference of a circle is proportional to its radius.