短路prep数学一题指教，已解决

yuantengxuanwu

from a group of 21 astronauts that includes 12 people with prvious experiece in space flight, a 3-person crew to be selected so that exactly 1 person in the crew has previous experience in space flight. How many different crews of this type are possible?
答案是432.
疑惑：我算的是12*9*8，864，为什么最后还要除以一个2啊，C9（2）=9*8没错的啊？
望大家指点

royzeiglercole

(C12/1)*(C9/2)=12*(9*8/2*1)=432

楼主，C9/2=9*8/2*1=36
A9/2才是9*8

yuantengxuanwu

(C12/1)*(C9/2)=12*(9*8/2*1)=432

楼主，C9/2=9*8/2*1=36
A9/2才是9*8

-- by 会员 royzeiglercole (2011/8/19 11:14:53)

谢谢啊，我之前这么对付的怎么全对了尼，亏了是来问了，谢谢，原来公式上记错了