there are so many ways to prove this

Here's the easiest one

Let's say you have a rectangle ABCD. AB is parallel to CD and BC is parallel to AD. And since rectangle has right angles it would be true that AB is congrudent to CD.

Therefore we can use a Pythagorean Theorem:

AB² + BC² = AC² and CD² + BC² = BD²

and since AB = CD -> AB² = CD² so

AB² + BC² = CD² + BC²

AB² + BC² = AB² + BC²

AC² = BD²