The Pythagorean theorem, or Pythagoras's theorem was know by the Babylonians in 2000 B.C - about 1500 years before the time of Pythagoras. 

In a right triangle, the sum of the squares of the lengths of the legs is equal to the square of the length of the hypotenuse.  This is a very simple and yet powerful theorem, which we can apply to many areas of life.  The proofs of this theorem are a little more complicated.  Perhaps the simplest is the geometrical proof, given about 1150 by the Indian mathematician Bhaskara - this is illustrated in the videos (right).