# Proportionality (mathematics)

Proportionality (mathematics)

In mathematics, two variables are proportional if a change in one is always accompanied by a change in the other, and if the changes are always related by use of a constant. The constant is called the coefficient of proportionality or proportionality constant. Alternatively, we can say that one of the variables is proportional to the other.

• If one variable is always the product of the other and a constant, the two are said to be directly proportional. x and y are directly proportional if the ratio is constant.
• If the product of the two variables is always equal to a constant, the two are said to be inversely proportional. x and y are inversely proportional if the product is constant.

If a linear function transforms 0, a and b into 0, c and d, and if the product a b c d is not zero, we say a and b are proportional to c and d. An equality of two ratios such as where no term is zero, is called a proportion.

### Other articles related to "proportionality":

Proportionality (mathematics) - Experimental Determination
... through the origin (0, 0), then the two variables are probably proportional, with the proportionalityconstant given by the line's slope ...