What was his theory?

Pythagoras most known discovernment was the Pythagorean Theorem. He discovered it not alone, but with all his group of followers but it is still called after him. This discovery is known all over the world and it is taught everywhere.

The Pythagorean Theorem basically is a relationship between the three sides of a right triangle. the equation for this relationship is a2+b2=c2. a and b are the shortest sides (legs) of the triangle and c is the longest side (hypotenuse). The equation means that axa + bxb = cxc. If you draw this you would get a right triangle with squares on each side. Here is an example.