The Pythagorean theorem is one of the best known formula in school mathmatics and the most important furmula in geometry. It's main thesis is, that the two shorter sides (cathesus) squared are equal to the the longest side (hypothenuse) squared.
a² + b² = c²
Here is one easy way to prove it without using numbers derivations making it extremly easy to understand. Also it's easy and fast to print.
For me it is one of the most vivid, self-explanatory demonstrations I experienced in math class. That is why I'm happy to share it with you.
If you want a triangle as middle piece, just print BigTriangle.stl once again. Enjoy!