This is a classic example of pigeon hole principle solution.

One of the archiving principles is:

Put (mn+ 1) objects into n drawers, and there must be at most (m+ 1) objects in a drawer.

Taking the midpoint of each side of a square, you can divide the square into four small squares, and these four small squares are four drawers. * * * There are 9 points, so there will be at least 3 in each square, so the area of the graph formed by these three points in the small square must be less than or equal to half of the small square, that is,1*1/4 *1/2 =1/8.