Consider a rectangle of height [i]h[/i] and width [i]w[/i] lying in the XY plane, with one corner at (0,0) and extending in the positive XY quadrant. If we take an element, it will have height h and length dw, giving an area of h*dw. Integrating from 0 [i]w[/i] gives us h*w as the area. My favorite way :)