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解排列组合问题时,有一口诀:相邻问题用捆绑,非邻问题用插空.本文用四例来介绍插空法.例1 马路上有一排十盏路灯,为节约用电,现要熄灭其中三盏,要求熄灭的三盏灯不在两端,且不相邻,一共有多少种不同的熄灭方法?分析组成空位的7盏亮灯之间没有顺序,三盏熄灭灯不在两端,所以7盏亮灯有6个空位.三盏熄灭的灯不相邻,用它们插空,即从6个空位中选出三个,同时,由于这三盏灯没有顺
When unmarshalling problems are combined, there is a slogan: adjacent problems are bundled, and non-neighbor problems are punctured. This article uses four cases to introduce the emptying method. Example 1 There is a row of ten street lights on the road. To save electricity, it is necessary to extinguish Among them, the three lights that are required to be extinguished are not at both ends, and they are not adjacent. How many different extinguishing methods are there? There is no order among the 7 lights that are composed of empty spaces, and three lights are not turned off at both ends, so The seven lights are six spaces. The three lights that are off are not adjacent, and they are used to empty the space, that is, three out of the six spaces are selected. At the same time, since the three lights are not