he invented pythagerous's theorem which says that if you draw a square on each side of a right angled triangle the area of the square on the hypotenuse (biggest side) will equal the area of the two other squares from the other sides added together ...always. this meant that you could find the length of any side if you did not have it. p.s he was also my maths teachers great great great great great great great grandfather not kidding !!!!!!!!!!