请问Prep一道组合题

springwitch

66.    7431-!-item-!-187;#058&005458
From a group of 21 astronauts that includes 12 people with previous experience in space flight, a 3-person crew is 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?

(A)  432

C1^12 C2^9 和C1^12*2C1^9是一个意思么？
如果是，具体怎么展开？我怎么算不出432呀？

请nn指正！多谢！

larichard

先从12个有经验的人中选择一个，从剩下9个没有经验的人当中选择2个，(12*9*8)/2=432

springwitch

先从12个有经验的人中选择一个，从剩下9个没有经验的人当中选择2个，(12*9*8)/2=432

-- by 会员 larichard (2012/12/23 21:00:53)

为什么要除以2呀？

springwitch

先从12个有经验的人中选择一个，从剩下9个没有经验的人当中选择2个，(12*9*8)/2=432

-- by 会员 larichard (2012/12/23 21:00:53)

为什么要除以2呀？

-- by 会员 springwitch (2012/12/24 0:19:25)

啊，懂了！多谢larichard