任教授在嗎?請教,這道題不會做,

If the sequence X1,X2,X3....Xn.....is such that X1=3,and Xn+1=2Xn-1,for n大於等於1,then X20-X19=多少?

答案是2的19次方,請問是怎麽計算出來的?書上說這是等比數列題,我還不會應用公司在這題上麵.

謝謝

所有跟帖： 

• 有括號嗎？ -lovchina- ♂ (82 bytes) () 01/10/2010 postreply 15:27:30

• 是後者 -陶陶樂,樂陶陶- ♀ (0 bytes) () 01/10/2010 postreply 17:29:26

• 題意不清，+1 和 -1 是和n 並行的嗎？ -任我為- ♂ (0 bytes) () 01/10/2010 postreply 15:47:41

• 抱歉,n+1和2Xn裏麵的n是平行的, -陶陶樂,樂陶陶- ♀ (19 bytes) () 01/10/2010 postreply 17:29:05

• 回複：任教授在嗎?請教,這道題不會做, -bearsback- ♂ (66 bytes) () 01/10/2010 postreply 16:02:43

• 恭喜老師 這麽快做出來。 -23731241- ♂ (0 bytes) () 01/10/2010 postreply 16:04:24

• 我還在想套等比數列的公式呢,暈... -陶陶樂,樂陶陶- ♀ (0 bytes) () 01/10/2010 postreply 21:39:41

• you are so smart! how did you get x[2]=5? -戲雨飛鷹- ♀ (276 bytes) () 01/10/2010 postreply 16:19:25

• 飛鷹同學，初二沒過關? 還不信我老師 -23731241- ♂ (0 bytes) () 01/10/2010 postreply 16:24:28

• haha, but I still don't know why x2=5:( -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 16:26:22

• correction: for n=19, X[20]-X[19] = 2^18(5-3) =2^19 -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 16:28:00

• sweetie, sweetie -bearsback- ♂ (44 bytes) () 01/10/2010 postreply 16:56:42

• 題目隻是給出x1=3, sequence,x[n+1]=2x[n-1], -戲雨飛鷹- ♀ (29 bytes) () 01/10/2010 postreply 16:58:21

• 回複：題目隻是給出x1=3, sequence,x[n+1]=2x[n-1], -bearsback- ♂ (35 bytes) () 01/10/2010 postreply 17:02:58

• 隻能是這樣了。要不就缺少條件了。不過， -戲雨飛鷹- ♀ (36 bytes) () 01/10/2010 postreply 17:08:38

• very true. -bearsback- ♂ (55 bytes) () 01/10/2010 postreply 17:30:00

• 考你一個難的（也很簡單）: -戲雨飛鷹- ♀ (59 bytes) () 01/10/2010 postreply 17:34:19

• 兩個2? -bearsback- ♂ (0 bytes) () 01/10/2010 postreply 17:36:55

• 回複：考你一個難的（也很簡單）: -bearsback- ♂ (22 bytes) () 01/10/2010 postreply 17:37:48

• 可以是任何操作，不局限在+－＊／ -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 17:40:16

• 回複：可以是任何操作，不局限在+－＊／ -bearsback- ♂ (64 bytes) () 01/10/2010 postreply 17:42:20

• wow!!!!!!!!!!!!!!!!!!!!!!!!! 佩服死了！ -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 17:44:26

• because of your hint (any operation) -bearsback- ♂ (0 bytes) () 01/10/2010 postreply 17:45:51

• 天才！我是看答案才知道的。88~~ -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 17:49:44

• sweetie, love makes me -bearsback- ♂ (70 bytes) () 01/10/2010 postreply 17:53:37

• 這是IBM一月（趣味智力）：二二得五 -戲雨飛鷹- ♀ (120 bytes) () 01/10/2010 postreply 18:01:10

• sweetie, ibm, hp, apple etc are not my type of company -bearsback- ♂ (23 bytes) () 01/10/2010 postreply 18:07:57

• haha，good one！"company"用的絕妙。 -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 18:31:11

• 裏麵的這個答案有點勉強：用exp運算 -戲雨飛鷹- ♀ (66 bytes) () 01/10/2010 postreply 19:05:36

• 回複：裏麵的這個答案有點勉強：用exp運算 -bearsback- ♂ (34 bytes) () 01/10/2010 postreply 19:21:07

• i'm wrong. the value of exp(log2) id not 10:) -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 19:29:45

• sweetie, your thinking is inspiring though the value is not ten -bearsback- ♂ (0 bytes) () 01/10/2010 postreply 20:16:07

• no, I copied the answer from 腦壇to show you. after posting -戲雨飛鷹- ♀ (38 bytes) () 01/10/2010 postreply 20:21:28

• oops, l meant left shift -bearsback- ♂ (52 bytes) () 01/10/2010 postreply 17:45:07

• 沒關係，左右兩個坑都要用：）：） -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 17:52:01

• hahhaha, that 坑 is the most important -bearsback- ♂ (17 bytes) () 01/10/2010 postreply 17:55:47

• 需要hint和話，說一聲。 -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 17:43:08

• 密碼，這個題目對你來講太簡單了。 -戲雨飛鷹- ♀ (0 bytes) () 01/10/2010 postreply 17:37:56