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 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.