The Egyptians knew that a triangle with sides 3, 4, and 5 make a 90^o angle. As a matter of fact, they had a rope with 12 evenly spaced knots like this one: The Egyptians knew that a triangle with sides 3, 4, and 5 make a 90^o angle. As a matter of fact, they had a rope with 12 evenly spaced knots like this one

they used to build perfect corners in their buildings and pyramids. It is believed that they only knew about the 3, 4, 5 triangle and not the general theorem that applies to all right triangles.