Let m,n be positive integers. An m x n chessboard is a rectangle of length m and height n that is divided into mn squares of length 1. For example, a stardard chessboard is 8 x 8. A rectangle on a chessboard, is a rectangle all of whose sides are made up of sides of the 1 x 1 squares.
How many rectangles are there on an m x n chessboard? For example, when m = n = 2, then the answer is 9.
I just don't know where to begin! Any help is greatly appreciated. Thanks.