Euclid wrote thirteen books of mathematics that have postulates, definitions, theorems and constructions, and mathematical proofs of the propositions mentioned. It is considered the most influencial textbook ever written in history.
Pythagoras came up with what is now called the Pythagorean Theorem (also known as "the Pythagoras' Theorem). This theorem is a fundamental relation among the three sides of a right triangle in Euclidean geometry.
