1本单元重、难点分析分类计数原理和分步计数原理是排列组合的核心内容,它既是推导排列数、组合数公式的基础,也是解决排列组合问题的重要方法.分类是把复杂的问题分解成互相排斥的几类,然后逐类解决,分步是把解决问题的方法分解成几个相互联系且相互独立的步骤,较复杂的排列组 1 The unit’s heavy and difficult analysis The principle of classification and counting and the step-by-step counting principle are the core elements of permutation and combination. It is not only the basis for the derivation of the array number and the combination number formula, but also an important method to solve the permutation and combination problems. The classification is to decompose the complex problems. To become a few types that are mutually exclusive, and then solve them class by class. The step-by-step method is to decompose the method of solving the problem into several interrelated and independent steps. The more complicated arrays are