Question

Explain how the formula for the area of a parallelogram is derived from that of a rectangle.

Please make it short and simple.

Answer 1

You can say that a rectangle is a special case of parallelogram such that the sides that give the height are perpindicular to the base. Because this is not the case you need to draw a dotted line from the bottom base vertice to the top base vertice to actually get the height. So, you can imagine basically pushing a rectangle so its side are no longer perpindicular.
http://www.mathgoodies.com/lessons/vol1/area_parallelogram.html