[Today’s run: none yet]
I fooled around some more with my large-triangle method. I found three equations describing the large triangle, but they all simplified down to the same thing and I could not see how to go any farther.
So I decided to try to find another method.
The unique feature of a right triangle is that one of the angles is 90 degrees. All triangles have angles that add up to 180 degrees. So, the pointy parts of a right triangle have to add up to be the other 90 degrees.
I saw that the big c-squared square needed to be part of the picture. If I arrange copies of my right triangle around the big square, it makes a larger square. The sides of the larger square are of length (B+A). There are two ways to describe the larger square. I set those against each other and did some algebra.
I don’t know exactly how I went wrong with my prior method. It seems like it should work. Maybe my three descriptions of the big triangle are not independent. Or something.