The cosine rule
The cosine rule is a fuller form of Pythagoras' Theorem.

Start with a general triangle, with the angles labelled P, Q & R and the sides opposite them labelled p, q & r like this:

Given two sides and the angle between them, the cosine rule allows you to calculate the length of the third side:

By rearranging the equations, as you would with Pythagoras, you can find any component. For example:

Or, if you know the three sides, you can find the angles. For instance:

Most recently updated at 18:29 (British time) on 07 April 2004