Pythagoras' theorem connects the lengths of two sides of a right-angled triangle with the length of the third side. It only works with right-angled triangles.

Pythagoras' theorem is

r^{2} = x^{2} + y^{2}

Where r is always the side opposite the right angle,

and x and y are the other two sides.

Phythagoras' theorem can be rearranged to find any of the three sides of a triangle. Remember that the length is not r^{2} but it is r (i.e. don't forget to root the answer).