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:
4 D+ ^0 j7 V1 i3 a8 @$ n- ~' M　　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!.
8 T4 X7 f- N  s　　Remember:
　　This does not apply if there are identical objects or ordering does not matter.
$ g) ?" \! {6 r9 ]: N! S　　Permutation with Selection
　　The number of ways n objects drawn from a collection of m distinct objects can be ordered is m!/(m-n)!.
  p. A; T9 {  g  G. \　　Example:
0 F( J1 k2 s2 V　　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.
　　Number of possible top ten list for 150 movies is 150!/(150-10)!.
　　Remember:
  c4 a: o0 R) m( P; e2 _　　This does not apply if there are identical objects or ordering does not matter