The plural of formula is formulae. A formula is a mathematical rule or relationship that uses letters to represent amounts which can be changed – these are called variables. For example, the formula to work out the area of a triangle.
(where represents the base of the triangle and represents the height of the triangle). and are variables because they can have different values, depending on the size of the triangle.
Formulae contain equals signs, as do equations, so it can sometimes be difficult to tell the difference between an equation and a formula.