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请问Prep一道组合题
66. 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！ |
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