# Equations, formulae and identities

**Expressions and identities**

An algebraic **expression **is not an equation, it is a combination of numbers, letters and operations. It also does not contain an equal sign.

For example:

4*x *+ 6 + 5*x *– 3

An **identity** is an expression re-wrieen in a equivalent form.

For example:

4*x *+ 5 + 2*x *– 2 ≡ 6*x + 3*

It can also be written with the ≡ or the = symbol.

**Equations**

An **equation** is an algebraic expression and a number, or two algebraic expressions with an **equals** sign. The equation also has a unknown which has a particular value, finding this value is also known as “solving” the equation.

For example:

*x* + 8 = 15

**Formulae**

A **formula** take equations a step further by linking **two** or more physical variables.

For example:

*P* = 2(*l *+* x*)