Re: [理工] [工數]-冪級數
※ 引述《ntust661 (661)》之銘言:
※ 引述《zendla (夏夜薄荷)》之銘言:
: 這題大概又是觀念問題了= =小弟觀念不好,不會判定nsin(1/n)是發散or收斂
: ,大大們幫個忙吧,題目如下
: 求以下級數收斂區間:
: ∞ 1 n
: Σ(1-nsin──)x
: n=1 n
: ans:[-1,1]
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 140.118.234.83
推
12/06 11:03,
12/06 11:03
→
12/06 11:03,
12/06 11:03
推
12/06 11:08,
12/06 11:08
→
12/06 11:10,
12/06 11:10
→
12/06 11:11,
12/06 11:11
→
12/06 11:13,
12/06 11:13
→
12/06 11:14,
12/06 11:14
不需要把sin展開
這整題只要用三角關係就可以做出來
lim符號省略
1-nsin(1/n)
-----------------
1-(n+1)sin(1/n+1)
-sin(1/n)+(1/n)cos(1/n)
-> ------------------------
-sin(1/n+1)+(1/(n+1))cos(1/(n+1)))
-> 1 = R
x=-1 你還要證明(1-nsin(1/n))隨n增大遞減才可以用交錯級數 過程trivial
f(n) = 1-nsin(1/n)
∞
x=1 級數 < f(1) + ∫f(x)dx = finite
1
所以收斂區間[-1,1]
→
12/06 11:40,
12/06 11:40
推
12/06 11:45,
12/06 11:45
0分
a_n -> 0 as n-> 00
不代表級數收斂
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 122.124.106.145
推
12/06 18:52, , 1F
12/06 18:52, 1F
→
12/06 18:54, , 2F
12/06 18:54, 2F
→
12/06 18:57, , 3F
12/06 18:57, 3F
→
12/06 18:58, , 4F
12/06 18:58, 4F
推
12/06 20:26, , 5F
12/06 20:26, 5F
推
12/06 21:32, , 6F
12/06 21:32, 6F
推
12/06 21:35, , 7F
12/06 21:35, 7F
推
12/06 21:47, , 8F
12/06 21:47, 8F
推
12/06 21:54, , 9F
12/06 21:54, 9F
推
12/06 21:57, , 10F
12/06 21:57, 10F
推
12/06 22:01, , 11F
12/06 22:01, 11F
→
12/07 18:11, , 12F
12/07 18:11, 12F
推
12/08 18:57, , 13F
12/08 18:57, 13F
討論串 (同標題文章)