Proportion

Proportion, in mathematics, equality of two ratios. The numbers a, b, c, and d are said to be in proportion if a/b = c/d. The expression may also be written as a:b::c:d. The term proportion is useful in describing the relationship between quantities whose ratio is constant—for example, the ratio between the radius and circumference of a circle. If there are two circles with circumferences c₁, c₂ and radii r₁ and r₂ then c₁ = 2πr, and c₂ = 2πr₂. This can be written c₁/r₁ = 2π = c₂/r₂. The figures c₁, r₁, C₂ and r₂ are in proportion, and we say that the circumference of a circle is proportional to its radius.

See also: Mathematics.