Li Yongle's 660 pays great attention to the understanding of the definition. At that time, I felt that it was a little simple to do 660 in the postgraduate entrance examination, but some of the difficulties were still quite large. But looking back on this, I still don't understand the definition deeply enough.

In addition, you may have read Tang Jiafeng's book before, and now you have to do Li Yongle's problem. This is normal. The problem may be that the two people's problem-solving ideas and understanding of postgraduate mathematics are different. You may be used to Tang Jiafeng's way of solving problems, but you are not used to Li Yongle's method of solving problems.

According to my classmates and I's postgraduate experience, I feel that Li Yongle has a good grasp of postgraduate mathematics. This is a matter of opinion. Insist on yourself and study hard is king. I wish you success in the postgraduate entrance examination.