Comparing the times of each item, you will find that there are m, m- 1, m-2, 0 and 4 times, while m-2 times the square of x and m-3 times the cube of x are all m times, so m=4 or 1+n=0. When m=4, it is found that it is not three terms, so m is not four terms, so n=- 1. At this time, there are still m-2 times x and m-3 times x left, so let m multiply y +0 times, that is, the times of m times x and m-2 times y are 4, that is to say, m=5. Finally, the formula * * * has three items, which are the general idea.