# Pythagoras' theorem

Source: https://projectgcse.co.uk/maths/shape_space/pythagoras_theorem

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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.

![](https://content.projectgcse.co.uk/img/maths/pythagoras.gif)

Pythagoras' theorem is
r2 = x2 + y2
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 r2 but it is r (i.e. don't forget to root the answer).