Study math for long enough and you will likely have cursed Pythagoras's name, or said "praise be to Pythagoras" if you're a bit of a fan of triangles.
But while Pythagoras was an important historical figure in the development of mathematics, he did not figure out the equation most associated with him (a2 + b2 = c2). In fact, there is an ancient Babylonian tablet (by the catchy name of IM 67118) which uses the Pythagorean theorem to solve the length of a diagonal inside a rectangle. The tablet, likely used for teaching, dates from 1770 BCE – centuries before Pythagoras was born in around 570 BCE.
It is. There's evidence of its use in the Old Babylonian period, evidence in 1800 B.C.E Egypt, India in 700-500 BCE, China during the Han Dynesty at least.
It's very simple to prove, and anywhere you find squares or triangles in architecture, it was used.