What is Pythagoras famous for?

Answer 1

Pythagoras's theorem, it's where you have a right-angled triangle and the squares of the 2 shortest sides added together make the square of the longest side, look up Pythagoras's theorem on Google for a more detail description.

Answer 2

Pythagoras is famous for his theorem that "in a right-angled triangle, the square on the hypotenuse is equal to the sum of the squares on the other two sides."

Basically, you have a right-angled triangle. The long side is the hypotenuse. Square it (times it by itself) and square the other two sides as well. The hypotenuse squared is equal to the other two sides squared.