Hi Bijan.
In your post #4, you got it yet !
The "divide and conquer approach" of your book is the fundamental theorem of calculus, a little hidden.
It is useful to draw the rectangular area B, with its four corners (x_{1},y_{1}) , (x_{1},y_{2}) , (x_{2},y_{1}) and (x_{2},y_{2}), and try to...