This one is a killer. If someone can prove it, i can get extra credit in my calc class!

"Prove that for any triangle, the inscribed rectangle with a base along one of the triangle's sides will have an area equal to half of the triangle's area."

http://www.vias.org/calculus/img/03_...ctions-233.gif

I know that it's easy to prove visually by folding a piece of paper to look like that above, but algebraically, i don't know how.

Thanks!