In one important special case, in which f (x, y) =0 for all values of x and y outside a rectangle having sides parallel to the x-axis,it is not actually necessary to check Eq. (3.5.7) for all values of x and y. In order to verify that X and Y are independent in this cast, it is sufficient to verify that Eq. (3.5.7.) is satisfied for all values of x and y inside the rectangle. In particular, let a, b, c,and d be given values such that -∞≤a