# Formulae

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.

$$\text{Triangle area} = \frac{bh}{2}$$ (where $$b$$ represents the base of the triangle and $$h$$ represents the height of the triangle). $$b$$ and $$h$$ 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.

• A formula shows the relationship between two or more variables. It is a calculation for a specific purpose, for example converting Fahrenheit to Celsius or vice versa. Subject to certain conditions, a formula is also always true, no matter what values are put in.
• An equation will usually only have one variable. Sometimes appearing more than once. It will only work for certain values, and is not always true. For example, the equation $$2x + 5 = 13$$ will only be true if $$x$$ is equal to 4. If any other value is put in, then the equation will no longer be correct.