排列和组合问题是组合数学的基础,其应用非常广泛,特别是它解题思路的独特性,对于培养能力和开发智力有着不可替代的作用.加法原理和乘法原理是两个最基本的计数原理,不仅排列组合公式要运用它们推导出来,而且许多与计数有关的问题也可以直接运用它们来解决.例1(2001年全国高中 The permutation and combination problem is the basis of combinatorial mathematics, its application is very extensive, especially the uniqueness of its problem-solving ideas, it plays an irreplaceable role in cultivating ability and developing intelligence. The addition principle and multiplication principle are the two most basic counting principles. Not only does the permutation of the combinatorial formulas are used to derive them, but many problems related to counting can also be directly applied to them. Example 1 (National High School in 2001)