新GRE数学要点:Permutation

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Permutation of Objects 　　The number of ways n distinct objects can be ordered is n!.
　　Example:
  A, D' k$ B1 b! J　　Number of ways 6 people can from a queue is 6!.
　　Number of ways 5 different cars can be parked in 5 parking spaces is 5!.
& z, B; _9 b) L1 W　　Remember:
9 M: B: [9 W- y# b& e　　This does not apply if there are identical objects or ordering does not matter.
　　Permutation with Selection
5 \8 J& _0 b$ ]9 X0 e　　The number of ways n objects drawn from a collection of m distinct objects can be ordered is m!/(m-n)!.
　　Example:
　　Number of ways a queue of length 3 can be formed from a group of 5 people is 5!/(5-3)!=5!/2!=5*4*3=60.
& D9 Q' \; E" S" A0 a* A8 [. a　　Number of possible top ten list for 150 movies is 150!/(150-10)!.
　　Remember:
　　This does not apply if there are identical objects or ordering does not matter