110(10n?1)n? =?910?19=
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?8,求?(n?1)(an?an?1)的值. [例16] 已知数列{an}:an?(n?1)(n?3)n?1解:∵ (n?1)(an?an?1)?8(n?1)[11?] (找通项及特征)
(n?1)(n?3)(n?2)(n?4) =8?[11?] (设制分组)
(n?2)(n?4)(n?3)(n?4) =4?(??1111?)?8(?) (裂项)
n?2n?4n?3n?4?1111?)?8?(?) (分组、裂项求和) ∴ ?(n?1)(an?an?1)?4?(n?4n?4n?1n?1n?2n?1n?3 =4?(? =
1311)?8? 4413 3提高练习:
1.已知数列?an?中,Sn是其前n项和,并且Sn?1?4an?2(n?1,2,L),a1?1,
⑴设数列bn?an?1?2an(n?1,2,??),求证:数列?bn?是等比数列; ⑵设数列cn?an,(n?1,2,??),求证:数列?cn?是等差数列; 2n
22.设二次方程anx-an+1x+1=0(n∈N)有两根α和β,且满足6α-2αβ+6β=3.
(1)试用an表示an?1;
*3.数列?an?中,a1?8,a4?2且满足an?2?2an?1?an n?N
⑴求数列?an?的通项公式;
⑵设Sn?|a1|?|a2|???|an|,求Sn;