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 24 cases: 246, 462, 624, 682,...)
从4个偶数中取3个偶数, do not think order first, you must use 组合 C4,3 = 4
four cases: (2,4,6), (2,4,8), (4,6,8), (2,6,8)
if you want to use your method, then keep your orders for 3个偶数,4个奇数.
you choose 3 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NOSorry. I made a mistake. I CANNOT say your method is wrong. 所以为C4,1*C3*1*C2,1　*　C5,1*C4,1*C3,1*C2,1　*　A7*7 但不等于100800.. if you want to keep your first two steps: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*C2,1 then you have already ordered your 3个偶数 and ordered your 4个奇数, so you MUST CHANGE your third step 将7个数排列为A7*7to "选 THREE POSITIONS OF 七位数 FOR YOUR 3个偶数. THIS HAVE C7,3cases "After you choosed 3 positions, you fill your 3个ORDERED偶数 into the positions FROM LEFT TO RIGHT one by one according to THE ORDERES and fill your 4个ODERED 奇数 into the remaining positions FROM LEFT TO RIGHT one by one ACCORDING TO THEIR ORDERS.So, the third step you haveC7,3choices. So, answer isC4,1*C3,1*C2,1 * C5,1*C4,1*C3,1*C2,1 * C7,3 = 100800. (writing as A4,3*A5,4*C7,3 is better because you consider ordering in your first two steps )Do not give up. There are many ways to do this kind of questions. For the second problem:Now, your method is even better. 从1,2,3,4,5,6,7中选2个奇数, keep order (first 奇数,second奇数), A4,2 (3,5 is not 5,3 now)从1,2,3,4,5,6,7中选2个偶数, keep order (first 偶数, seond 偶数), A3,2Now you choose two positions of 四位数 for your 2个奇数. Your SECOND 奇数 must be 个位数,and your FIRST 奇数 have 3 positions (十位,百位,千位) to choose; C3,1.After you finish to fill in your奇数, you fill your 偶数 into the remaining positions IN ORDER .(from left to right, you fill in your FIRST偶数 , your SECOND偶数). So there are total C3,1 choices.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DO not give 不要放弃 那个是我的某东西的密码- -用汉语说一遍好吗My English is too terrible..May you help me,please?(啊啊啊 指定很多语法错误..)sorry, my computer has no chinese software. i can not type chinese here. example, 从{2,4,6,8} 有序选2, 8, 6从{1,3,5,7,9} 有序选 5, 3, 9,1 now, you want to form a 七位数 abcdefg then u need to find 3 positions for 你的偶数, example, c, d,f. Thus, after you add 你的有序偶数 2,8,6 to the 3 positions c,d,f, you obtain "ab28e6g".Then you add 你的有序奇数 5, 3, 9, 1 to the remaining 4 pisitions a, b, e, g, you obtain a 七位数 5328961. 因为你进行了排序 in the first two steps, you do not need 排序 again in the third step, only consider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,...) tell me你选的三个偶数, 四个奇数, 三个位数啊啊啊啊啊(3)不明白- -tell me你选的三个偶数, 四个奇数, 三个位数(abcdefg 选三个letters )