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GRE数学考点：set、排列、组合SetAsetisacollectionofobjectsorthings.Eachobjectinasetamemberorelementofthatset.Sizeofasetisthenumberofmembersintheset.Example:Thesetofevennumbersbetween2and10isofsize5:{2,4,6,8,10}.Thesetofprimesbetween2and10isofsize4:{2,3,5,7}.Remember:EachmemberofsetAbelongstoAorisinthesetA.Asetcannothaverepeatingmember:{1,3,1,2}isnotaset.Rearrangingtheorderofthemembersdoesnotchangetheset:{1,2,3}issameas{3,2,1}.CombinationsNumberofwayswecanselectnobjectsfromagroupofmdistinctobjectsism!/(n!*(m-n)!).Example:Numberofpossiblewaystopick4oscarnominationsfromalistof50moviesis50!/(4!*(50-4)!)=50!/(4!*46!)=50*49*48*47/24=230300.Remember:Thisdoesnotapplyifthereareidenticalobjects.PermutationofObjectsThenumberofwaysndistinctobjectscanbeorderedisn!.Example:Numberofways6peoplecanfromaqueueis6!.Numberofways5differentcarscanbeparkedin5parkingspacesis5!.Remember:Thisdoesnotapplyifthereareidenticalobjectsororderingdoesnotmatter.
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