题目内容
20.有5女5男共10个同学一起到圆明园游玩,老师给他们照相时,要求他们站成一排,男生和男生不相邻,女生和女生不相邻,那么同学们共有多少种排队方式?分析 先排男生有${A}_{5}^{5}$种排法,形成了6个空,让女生插空共有${A}_{6}^{5}$种排法,
解答 解:${A}_{5}^{5}$×${A}_{6}^{5}$
=(5×4×3×2×1)×(6×5×4×3×2)
=120×720
=86400(种)
答:同学们共有86400种排队方式.
点评 本题考查了排列组合知识,当考查不相邻问题时,利用“插空法”解答简洁.
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6.3+7= | 4.8÷0.08= | 9+0.1= | 3-0.67-0.33= |
0.08÷0.2= | 0.2×0.4= | 1÷0.05= | 0+2÷5= |
0.081×100= | 7.96×10= | 40万-13万= | 0.45+3.5+.055= |
1-0.1= | 0.31+0.3= | 0.72÷1.2= | 0.05×1= |
4.2+5.08= | 7.2÷0.072= | 4.3-3= | 0.75÷1.5= |
1.05-0.95= | 2.5×4= | 2.5×2.4= | 3.2-1.34-1.66= |
12.8-0.28= | 1.25×0.8= | 5.6-5.6÷7= | 3.60.7÷3.6×0.7= |
0.8+0.2-0.8+0.2= | 1÷1.25÷4= | 8.4×5= | 0×9.56+7.2= |