Here are some proofs, because why not?
For all right triangles, the square of the hypotenuse is equal to the sum of the squares of the legs. See also: Trigonometry Proofs
For any polygon, the sum of its interior angles is always equal to (n-2) * 180, where n is the number of sides. See also: Polygons Angles
Same as Proof 1, because I'm a mathematician of discerning taste. See also: Trigonometry Proofs
That's all the proofs I have for now, folks. Next