gwd 24套 math第一套25题 想半天了。。求解

raindrop琼

Q25:

A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.How many such arrangements of the 6 people are possible?

A.5

B.6

C.9

D.24

E.36

一个个试 能找出是5种 但比较严谨的数学思路因该是怎样的呢

还有大家知道 哪有gwd24套数学题详解 或 讨论帖合集么 谢啦

henry881124

设身高分别为1-6
根据题意，1只能在左下而6只能在右上
讨论2的情况：
若2在左上，则3必在下中，剩下的4 5位置可对换，计两种情况
若2在下中，则3可在右下，此时4 5位置确定，一种情况；若3在左上，则4 5可换位，两种情况
共计2+1+2=5种情况

raindrop琼

谢谢了~