As shown in the figure, in a big square with a side length of less than 8cm, put three sheets with an area of 25cm & sup 2；; There are three square pieces of paper, red, yellow and blue. The area of this big square covered by three pieces of paper is 45cm &；; What is the area of this big square?

The covered part is a piece of blue, plus two pieces of red and yellow with the same area (because of the side length of the square, etc.) (45-25)/2= 10, both red and yellow are 10 square centimeter 25=5*5, the side length of three pieces is 5, and the other side of red and yellow is10.