1. Be familiar with "addition theorem (classification counting principle)" and "multiplication theorem (step counting principle)".

This is the basis of analyzing permutation and combination problems.

2. Master the "special position method" and "special element method". That is, whether the requirements of special "location" are met first in the analysis process, or

First, meet the requirements of special "elements".

3. Familiar with "direct method" and "indirect method". If the direct analysis is complicated, the indirect method is adopted. In other words, no conditions are attached.

Count the number of species, and then subtract those that don't meet the meaning of the question.

4. Pay attention to the classification of "no weight" and "no leakage" during analysis.

5. Understand that "arrangement is related to order" and "combination has nothing to do with order".

6. Pay attention to the memory of several special questions.