Thread:Balu.ur/@comment-25501845-20141116152623/@comment-24808864-20141116154502

From youtube, Shaunteaches has a decent video for it here.

You should follow his explanation slowly (it only takes two and a half minutes) and copy it on separate paper. But using your variables, replace 3 with l, 4 with b, and 12 with h.

Technically, when he starts using "5", you should be use $$ \sqrt{l^2+b^2}$$ in place of 5, but the argument he gives in the video is a great illustration of the geometry of the problem.

You can remember it as a "double" Pythagorean Theorem proof!