It is better to see this property if you stay with paralelograms and use the Cavalieri Principle. There is also a demonstration of the first formula using triangles and again using only Cavalieri. I think I can find it wait a second.

It is better to see this property if you stay with parallelograms and use the Cavalieri Principle. Here is page with an animation showing what I mean (clink on the link and accept).

There is also a demonstration of the first formula using triangles and again using only Cavalieri. There was an animation "proof without words" for it in the site "art of problem solving" but I can't find it anymore.