# Proportionality

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.

A proportionality relationship happens when two quantities or numbers x and y are related multiplicatively by a fixed number. This can occur when either their ratio x/y is a fixed number (direct proportionality), or their product xy is a fixed number (inverse proportionality). If x is directly proportional to y, then we write ${\displaystyle x\propto y}$.[1] The fixed number of a proportionality relationship is called the constant of proportionality.[2][3]

## References

1. "Compendium of Mathematical Symbols". Math Vault. 2020-03-01. Retrieved 2020-09-20.
2. Weisstein, Eric W. "Proportional". mathworld.wolfram.com. Retrieved 2020-09-20.
3. "Directly Proportional and Inversely Proportional". www.mathsisfun.com. Retrieved 2020-09-20.