Pythagorean Theorem (500 BC)

Theorem: In any right triangle, the sum of the squares of the shorter sides is equal to the square of the longest side, called the hypotenuse.

Author: Pythagorus is officially attributed, around 500 BC, although some historians think it likely that it was first discovered by one of his followers instead.

Importance: The Pythagorean serves to this day as one of the first theorems examined in plane geometry. Legend has it that early builders used a close loop of rope that was 15 distance units long, with marks at an initial point, then 3 units beyond, and another 4 units beyond that. This loop would then be used to check whether a given corner was square.