但是先头我是这么想的,从4个偶数中取3个偶数,那就为C4,1*C3*1*C2,1

　　错

　　C4,1*C3,1*C2,1=4*3*2是排列24cases:246,462,624,682,...)

　　从4个偶数中取3个偶数,donotthinkorderfirst,youmustuse组合C4,3=4

　　fourcases:(2,4,6),(2,4,8),(4,6,8),(2,6,8)

　　ifyouwanttouseyourmethod,thenkeepyourordersfor3个偶数,4个奇数.

　　youchoose3位for七位数.

　　所以为C4,1*C3*1*C2,1　*　C5,1*C4,1*C3,1*C2,1　*　C7,3=100800..

　　呃..那的确我也看出来了可是我做的另外一个题也是用的错误的方法也解出来了是怎么回事啊从1,2,3,4,5,6,7中任选2个奇数和2个偶数,组成没有重复数字的四位数,其中奇数个数为?个位数为A4,1则十位,百位,千位可为A3,1(剩下3个奇数中1个),A3,1(3个偶数中的一个),A2,1(3个偶数中的一个),所以为A4,1*A3,3*A3,1*A3,1*A2,1结果也跟答案对上了啊帮我解释下怎么回事....我不太明白那个题也是这么做的对了为什么这个NO

　　Sorry.Imadeamistake.ICANNOTsayyourmethodiswrong.所以为C4,1*C3*1*C2,1　*　C5,1*C4,1*C3,1*C2,1　*　A7*7但不等于100800..ifyouwanttokeepyourfirsttwosteps:但是先头我是这么想的,从4个偶数中取3个偶数,那就为C4,1*C3*1*C2,1从5个奇数中去4个奇数,那就为C5,1*C4,1*C3,1*C2,1thenyouhavealreadyorderedyour3个偶数andorderedyour4个奇数,soyouMUSTCHANGEyourthirdstep将7个数排列为A7*7to"选THREEPOSITIONSOF七位数FORYOUR3个偶数.THISHAVEC7,3cases"Afteryouchoosed3positions,youfillyour3个ORDERED偶数intothepositionsFROMLEFTTORIGHTonebyoneaccordingtoTHEORDERESandfillyour4个ODERED奇数intotheremainingpositionsFROMLEFTTORIGHTonebyoneACCORDINGTOTHEIRORDERS.So,thethirdstepyouhaveC7,3choices.So,answerisC4,1*C3,1*C2,1*C5,1*C4,1*C3,1*C2,1*C7,3=100800.(writingasA4,3*A5,4*C7,3isbetterbecauseyouconsiderorderinginyourfirsttwosteps)Donotgiveup.Therearemanywaystodothiskindofquestions.Forthesecondproblem:Now,yourmethodisevenbetter.从1,2,3,4,5,6,7中选2个奇数,keeporder(first奇数,second奇数),A4,2(3,5isnot5,3now)从1,2,3,4,5,6,7中选2个偶数,keeporder(first偶数,seond偶数),A3,2Nowyouchoosetwopositionsof四位数foryour2个奇数.YourSECOND奇数mustbe个位数,andyourFIRST奇数have3positions(十位,百位,千位)tochoose;C3,1.Afteryoufinishtofillinyour奇数,youfillyour偶数intotheremainingpositionsINORDER.(fromlefttoright,youfillinyourFIRST偶数,yourSECOND偶数).SotherearetotalC3,1choices.So,answer=A4,2*A3,2*C3,1

　　啊啊啊啊还是不懂..可是为什么,C7.3怎么出来的..我就听懂你说的DOnotgive不要放弃那个是我的某东西的密码--用汉语说一遍好吗MyEnglishistooterrible..Mayyouhelpme,please?(啊啊啊指定很多语法错误..)

　　sorry,mycomputerhasnochinesesoftware.icannottypechinesehere.example,从{2,4,6,8}有序选2,8,6从{1,3,5,7,9}有序选5,3,9,1now,youwanttoforma七位数abcdefgthenuneedtofind3positionsfor你的偶数,example,c,d,f.Thus,afteryouadd你的有序偶数2,8,6tothe3positionsc,d,f,youobtain"ab28e6g".Thenyouadd你的有序奇数5,3,9,1totheremaining4pisitionsa,b,e,g,youobtaina七位数5328961.因为你进行了排序inthefirsttwosteps,youdonotneed排序againinthethirdstep,onlyconsider你选的三个偶数的位数(这有C(7,3)个组合).

　　还是不明白哦T.T

　　你(1)从2,4,6,8中选三个偶数(2)从1,,3,5,7,9中选四个奇数,(3)从七个位数abcdefg中选三个位数(g-个位,f-十位,e-百位,d-千位,...)tellme你选的三个偶数,四个奇数,三个位数

　　啊啊啊啊啊(3)不明白--

　　tellme你选的三个偶数,四个奇数,三个位数(abcdefg选三个letters)