you are so smart! how did you get x[2]=5?

I use generalizing...

X[n+1]- X[n]= 2(X[n]- X[n-1])

thus,

X[n+1]- X[n]
=2*2(X[n-1] - X[n-2])
=2^3(X[n-2] - X[n-3])
=2^4(X[n-3] - X[n-4])
....
=2^(n-1)(X[2] - X[1])

for X1=3 Thus, X2=???


for n=19, X[20]-X[19] = 2^19(?-3)

所有跟帖： 

• 飞鹰同学，初二没过关? 还不信我老师 -23731241- ♂ (0 bytes) () 01/10/2010 postreply 16:24:28

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

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

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

• 题目只是给出x1=3, sequence,x[n+1]=2x[n-1], -戏雨飞鹰- ♀ (29 bytes) () 01/10/2010 postreply 16:58:21

• 回复：题目只是给出x1=3, sequence,x[n+1]=2x[n-1], -bearsback- ♂ (35 bytes) () 01/10/2010 postreply 17:02:58

• 只能是这样了。要不就缺少条件了。不过， -戏雨飞鹰- ♀ (36 bytes) () 01/10/2010 postreply 17:08:38

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

• 考你一个难的（也很简单）: -戏雨飞鹰- ♀ (59 bytes) () 01/10/2010 postreply 17:34:19

• 两个2? -bearsback- ♂ (0 bytes) () 01/10/2010 postreply 17:36:55

• 回复：考你一个难的（也很简单）: -bearsback- ♂ (22 bytes) () 01/10/2010 postreply 17:37:48

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

• 回复：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！ -戏雨飞鹰- ♀ (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~~ -戏雨飞鹰- ♀ (0 bytes) () 01/10/2010 postreply 17:49:44

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

• 这是IBM一月（趣味智力）：二二得五 -戏雨飞鹰- ♀ (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"用的绝妙。 -戏雨飞鹰- ♀ (0 bytes) () 01/10/2010 postreply 18:31:11

• 里面的这个答案有点勉强：用exp运算 -戏雨飞鹰- ♀ (66 bytes) () 01/10/2010 postreply 19:05:36

• 回复：里面的这个答案有点勉强：用exp运算 -bearsback- ♂ (34 bytes) () 01/10/2010 postreply 19:21:07

• i'm wrong. the value of exp(log2) id not 10:) -戏雨飞鹰- ♀ (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 脑坛to show you. after posting -戏雨飞鹰- ♀ (38 bytes) () 01/10/2010 postreply 20:21:28

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

• 没关系，左右两个坑都要用：）：） -戏雨飞鹰- ♀ (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和话，说一声。 -戏雨飞鹰- ♀ (0 bytes) () 01/10/2010 postreply 17:43:08

• 密码，这个题目对你来讲太简单了。 -戏雨飞鹰- ♀ (0 bytes) () 01/10/2010 postreply 17:37:56