排列组合是高中数学的重点内容,有着非常广泛的应用．其中有一类特殊的计数问题,既无法用两大计数原理解决,也很难用排列的知识找到解答,它们有一个共同的特征,就是问题的答案可以用一个组合数来表示．这里面究竟有何奥妙呢?通过本文希望能给大家一个解答． Arrangement and combination are the focus of high school mathematics and have a very wide range of applications. There is a special kind of counting problem, which can not be solved by the two major counting principles, and it is difficult to find the solutions with arranged knowledge. They have a common feature, that is, the answers to the questions can be represented by a combination number. What’s the magic in it? Through this article we hope to give you a solution.