论文部分内容阅读
有些数学关系既不易理解也不易记忆,但是把它和准确、形象、生动的实例联系在一起,困难便消失了。组合数的两个性质就是这样。C_n~m=C_n~(n-m)表示从n个元素里挑m个元素出来和挑n-m个元素留下是一回事。公式C_n~m=C_(n-1)~m+C_(n-1)~(m-1)表示从n个元素中挑m个元素可以分两种情况。不挑元素A的有C_(n-1)~m种,一定挑元素A的有C_(n-1)~(m-1)种。“无A”、“有A”是这个公式的“题眼”,抓住“题眼”,问题就迎刃而解了。 C_n~m=C_(n-1)~m+C_(n-1)~(m-1)和C_n~m=C_n~(n-m)分别表达了
Some mathematical relationships are neither easy to understand nor easy to remember, but linking it to accurate, vivid and vivid examples, the difficulty disappears. This is the two properties of the combination number. C_n~m=C_n~(n-m) means that picking out m elements out of n elements is the same as picking n-m elements. The formula C_n~m=C_(n-1)~m+C_(n-1)~(m-1) indicates that there are two cases in which m elements can be selected from n elements. There are C_(n-1)~m species that do not pick element A, and C_(n-1)~(m-1) species that must pick element A. “No A” or “A” is the “eye” of this formula. If you grasp the “eye”, the problem will be solved. C_n~m=C_(n-1)~m+C_(n-1)~(m-1) and C_n~m=C_n~(n-m) are expressed separately