A rectangle ABCD is given. On the side AB, n different points are chosen strictly between A and B. Similarly, m different points are chosen on the side AD. Lines are drawn from the points parallel to the sides. How many rectangles are formed in this way? (One possibility is shown in the figure.)
https://cdn.artofproblemsolving.com/attachments/0/1/dcf48e4ce318fdcb8c7088a34fac226e26e246.png combinatorial geometrycombinatorics