GRE数学重要知识点：排列

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　　Permutation of Objects

　　The number of ways n distinct objects can be ordered is n!.

　　Example:

　　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!.

　　Remember:

　　This does not apply if there are identical objects or ordering does not matter.

　　Permutation with Selection

　　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.

　　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.