Find a general formula involving and for the total number of rectangles. There is a second hint in the details, but do not look too quickly—thinking about the solution is more important than the answer. Note that a square is a special kind of rectangle, so count them too.
Hint 2: Choose an horizontal rectangle on the bottom and a vertical rectangle on the left. Add all the rectangles above to form a green strip across the big rectangle . Similarly, add all the rectangles to the right of to form a red strip across . The two strips overlap to form a blue rectangle . and determine uniquely and vice versa. To find how many ways there are to form a blue rectangle, count the number of pairs .
Hint 3: In a row of length there are rectangles.
This problem generalizes to how many rectangular sub-boxes can be made from a three-dimensional rectangular box of dimensions , where , , and are positive integers. Sketch a hint and derive the formula.