GRE重要知識點小結整理

　　Set

　　A set is a collection of objects or things. Each object in a set a member or element of that set.Size of a set is the number of members in the set.

　　Example:

　　The set of even numbers between 2 and 10 is of size 5:{2,4,6,8,10}.

　　The set of primes between 2 and 10 is of size 4:{2,3,5,7}.

　　Remember:

　　Each member of set A belongs to A or is in the set A.

　　A set can not have repeating member:{1,3,1,2}is not a set.

　　Rearranging the order of the members does not change the set:{1,2,3}is same as{3,2,1}.

　　Combinations

　　Number of ways we can select n objects from a group of m distinct objects is m!/!).

　　Example:

　　Number of possible ways to pick 4 oscar nominations from a list of 50 movies is 50!/!)=50!/=50494847/24=230300.

　　Remember:

　　This does not apply if there are identical objects.

　　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.

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