in how many ways can 4 adults and 5 children be arranged in a single line under each condition

题目

in how many ways can 4 adults and 5 children be arranged in a single line under each condition
1.children together and adults together 2 all the adults are together
为什么1*2*3*4*5*1*2*3*4*2不再乘个4呢？
可以横着排竖着排左斜着排右斜着排啊~

答案

这个就是排列组合啊~1.先把4大人和5个孩子看做整体,这两个整体就可以全排列了,也就是2!,然后4个大人之间也是可以互相换位子,因此四个大人也要全排列,也就是4!,以此类推,孩子也一样全排列5!最终,式子是：2!*4!*5!=2*1...