The Pythagorean Theorem or Pythagoras Theorem
a^2 +b^2 = c^2
is "The square on the hypotenuse is equal to the sum of the squares of the other two sides," where the hypotenuse is the longest side.
Pythagoras made many connections between the universe and geometry and, using his knowledge from the study of geometry, he was able to prove the connection between a square and right-angled triangles (those with an angle of 90 degrees). The belief was already held in Babylon and other ancient cities of academic genius- Greece for example, from where Pythagoras originated, but Pythagoras used it mathematically and is therefore attributed with its discovery.