但是先頭我是這么想的,從4個偶數(shù)中取3個偶數(shù),那就為C4,1*C3*1*C2,1
錯
C4,1 * C3,1 * C2,1 = 4*3*2 是排列 24 cases: 246, 462, 624, 682,...)
從4個偶數(shù)中取3個偶數(shù), do not think order first, you must use 組合 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個偶數(shù),4個奇數(shù).
you choose 3 位 for 七位數(shù).
所以為C4,1*C3*1*C2,1　*　C5,1*C4,1*C3,1*C2,1　*　C7,3= 100800..呃.. 那的確 我也看出來了 可是我做的另外一個題也是用的錯誤的方法 也解出來了 是怎么回事啊從1,2,3,4,5,6,7中任選2個奇數(shù)和2個偶數(shù),組成沒有重復數(shù)字的四位數(shù),其中奇數(shù)個數(shù)為?個位數(shù)為A4,1 則十位,百位,千位可為A3,1(剩下3個奇數(shù)中1個),A3,1(3個偶數(shù)中的一個),A2,1(3個偶數(shù)中的一個),所以為A4,1*A3,3*A3,1*A3,1*A2,1 結(jié)果也跟答案對上了啊 幫我解釋下怎么回事....我不太明白 那個題也是這么做的 對了 為什么這個NOSorry. I made a mistake. I CANNOT say your method is wrong. 所以為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:但是先頭我是這么想的,從4個偶數(shù)中取3個偶數(shù),那就為C4,1*C3*1*C2,1從5個奇數(shù)中去4個奇數(shù),那就為C5,1*C4,1*C3,1*C2,1 then you have already ordered your 3個偶數(shù) and ordered your 4個奇數(shù), so you MUST CHANGE your third step 將7個數(shù)排列為A7*7to "選 THREE POSITIONS OF 七位數(shù) FOR YOUR 3個偶數(shù). THIS HAVE C7,3cases "After you choosed 3 positions, you fill your 3個ORDERED偶數(shù) into the positions FROM LEFT TO RIGHT one by one according to THE ORDERES and fill your 4個ODERED 奇數(shù) 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. 從1,2,3,4,5,6,7中選2個奇數(shù), keep order (first 奇數(shù),second奇數(shù)), A4,2 (3,5 is not 5,3 now)從1,2,3,4,5,6,7中選2個偶數(shù), keep order (first 偶數(shù), seond 偶數(shù)), A3,2Now you choose two positions of 四位數(shù) for your 2個奇數(shù). Your SECOND 奇數(shù) must be 個位數(shù),and your FIRST 奇數(shù) have 3 positions (十位,百位,千位) to choose; C3,1.After you finish to fill in your奇數(shù), you fill your 偶數(shù) into the remaining positions IN ORDER .(from left to right, you fill in your FIRST偶數(shù) , your SECOND偶數(shù)). So there are total C3,1 choices.So, answer = A4,2*A3,2*C3,1啊啊啊啊 還是不懂.. 可是為什么,C7.3怎么出來的..我就聽懂你說的DO not give 不要放棄 那個是我的某東西的密碼- -用漢語說一遍好嗎My English is too terrible..May you help me,please?(啊啊啊 指定很多語法錯誤..)sorry, my computer has no chinese software. i can not type chinese here. example, 從{2,4,6,8} 有序選2, 8, 6從{1,3,5,7,9} 有序選 5, 3, 9,1 now, you want to form a 七位數(shù) abcdefg then u need to find 3 positions for 你的偶數(shù), example, c, d,f. Thus, after you add 你的有序偶數(shù) 2,8,6 to the 3 positions c,d,f, you obtain "ab28e6g".Then you add 你的有序奇數(shù) 5, 3, 9, 1 to the remaining 4 pisitions a, b, e, g, you obtain a 七位數(shù) 5328961. 因為你進行了排序 in the first two steps, you do not need 排序 again in the third step, only consider你選的三個偶數(shù)的位數(shù) (這有 C(7,3)個 組合).還是不明白哦T.T你(1) 從 2,4,6,8 中選三個偶數(shù)(2) 從1,,3,5,7,9 中選四個奇數(shù),(3) 從七個位數(shù) abcdefg 中選三個位數(shù) (g-個位, f-十位, e-百位, d-千位,...) tell me你選的三個偶數(shù), 四個奇數(shù), 三個位數(shù)啊啊啊啊啊(3)不明白- -tell me你選的三個偶數(shù), 四個奇數(shù), 三個位數(shù)(abcdefg 選三個letters )