众所周知求解排列、组合题有两大类探索思路 :直接法和间接法 ,间接法中一般都用排除法 ,而在排除法中又比较习惯于用“相减排除法”———从总数中减去不符合题目要求的 .课本上是这么写的 ,老师也是这么教的 .然而学生在实际做题中经常遇到一类题 ,用直接法是非常繁琐的 ,用 As we all know, there are two major types of exploration ideas for solving permutations and combinatorial questions: direct and indirect methods. Indirect methods generally use exclusion methods, and in exclusion methods, they are more accustomed to using “subtractive elimination methods”—from the total number. Subtraction of textbooks that do not meet the requirements of the title is written in this way, and teachers are also taught this way. However, students often encounter a class of questions in the actual question, using the direct method is very tedious, with