One of the joys of getting old is that you forget things.   So one of the things I recall is that the converse of the Pythagorean Theorem is true, that is, if a triangle with short sides a and b and long side c is such that

a² + b² = c²,

then the triangle must be a right triangle with the angle between sides a and b being 90°.  But I didn’t recall how to prove it.  So I thought I would see if I could do it without looking up any sources.

See the Pythagorean Theorem Converse