GRE数学考试重要知识点英文版

　　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!/(n!*(m-n)!).
　　Example:
　　Number of possible ways to pick 4 oscar nominations from a list of 50 movies is 50!/(4!*(50-4)!)=50!/(4!*46!)=50*49*48*47/24=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.