Pythagoras discovered that the ancient Egyptians used a 3:4:5 right triangle to build the pyramids. He investigated this further and deduced the Pythagorean theorem, which states that for any right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. Pythagoras proved this by drawing squares on each side of right triangles and showing that the areas added up. The Pythagorean theorem has been used since ancient times in architecture, engineering, and more recently in technology like screens.