# Equation

An equation is a mathematical statement that two things are equal. It consists of two expressions, one on each side of an 'equals' sign. For example: This equation states that 12 is equal to the sum of 7 and 5, which is obviously true. In an equation, the left side is always equal to the right side.

## Using variables

The most common equations contain one or more variables. If we let x stand for an unknown number, and write the equation We know the left side and right side are equal, so we can see that x must be 12+5 or 17. This is the only value that x can have that makes the equation a true statement. We say that x=17 'satisfies' the equation.

This process of finding the value of the unknowns is called "solving the equation". We often say that we "solve for x" - meaning solve the equation to find the value of the unknown number x.

## A common mistake

You will often see things that are called equations when they are not. For example you may see something like this referred to as an equation: It does not have an equals sign in it, and so is not an equation. It is called an 'expression'.

## Algebra

The study of algebra is in large part about learning ways to solve various kinds equations. For example, there are ways to solve quadratic equations such as