**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.

Read more about Proportionality (mathematics): Geometric Illustration, Symbol, Direct Proportionality, Inverse Proportionality, Hyperbolic Coordinates, Exponential and Logarithmic Proportionality, Experimental Determination

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