# 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

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