2022年数列与数学归纳法专题2 .pdf
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1、数列与数学归纳法专题上海市久隆模范中学石英丽经典例题【例 1】已知数列na的前n项和为nS,且*,855NnanSnn.(1)证明:1na是等比数列;(2)求数列nS的通项公式,并求出使得nnSS1成立的最小正整数n.解:(1)当1n时,141a;当2n时,15511nnnnnaaSSa,所以16511nnaa.又01511a,所以数列1na是以 15 为首项,65为公比的等比数列.(2)由(1)知:165151nna,得1651nna从而*1,906575NnnSnn;由nnSS1得252651n,9.141252log65n,最小正整数15n.【例 2】等差数列na的前n项和为239,21
2、,31SaSn(1)求数列na的通项na与前n项和nS;(2)设()nnSbnnN,求证:数列nb中任意不同的三项都不可能成为等比数列解:(1)由已知得11213393 2aad,2d,故212(2)nnanSn n,(2)由()得2nnSbnn假设数列nb中存在三项pqrbbb,(pqr,互不相等)成等比数列,则2qprbb b即2(2)(2)(2)qpr2()(2)20qprqprpqrNQ,2020qprqpr,22()02prprprpr,与pr矛盾所以数列nb中任意不同的三项都不可能成等比数列【例3】已 知 公 差 不 为0 的 等 差 数 列na的 首 项1a为aRa,设 数 列
3、的 前n 项 和 为4211,1,1,aaaSn且成等比数列.(1)求数列na的通项公式及nS;(2)记naaaaBSSSAnnn2221211111,1112,当2n时,试比较nA与nB的大小解:(1)设等差数列na的公差为d,由4122111aaa,得)3(1121daada.因为0d,所以ad所以21,1nanSnaann.(2)因为11121nnaSn,所以)111(211121naSSSAnn.因为aann1221,所以nnnaaaaaaBn211221121111111122221.当12,210nCCCnnnnnn时,即nn211111.所以,当nnnnBAaBAa时当时0;0.
4、【例 4】已知21a,点1,nnaa在函数xxxf22的图象上,其中=1,2,3,(1)证明数列na1lg是等比数列;(2)设nnaaaT11121,求nT及数列na的通项;(3)记211nnnaab,求数列nb的前项和Sn,并证明132nnTS=1.文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4
5、S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8
6、K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4
7、S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8
8、K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4
9、S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8
10、K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3解:(1)由已知212nnnaaa,211(1)nnaa12aQ11na,两边取对数得1lg(1)2lg(1
11、)nnaa,即1lg(1)2lg(1)nnaalg(1)na是公比为2 的等比数列.(2)由()知11lg(1)2lg(1)nnaa1122lg3lg3nn1213nna(*)12(1)(1)nTaan(1+a)012222333n-12 321 223n-1+2=n2-13由(*)式得1231nna(3)nnnaaa2211(2)nnnaaa11111()22nnnaaa11122nnnaaa.又112nnnbaa1112()nnnbaa12nSbbn+b122311111112()nnaaaaaa+11112()naa.1221131,2,31nnnnaaaQ22131nnS.又213nn
12、T2131nnST.【例 5】已知数列na满足2,021aa,且对任意*,Nnm都有211212)(22nmaaanmnm.(1)求53,aa;(2)设)(*1212Nnaabnnn,证明:nb是等差数列;(3)设*11,0,Nnqqaacnnnn,求数列nc的前n项和nS.解:(1)由题意,6221,2123aaanm可得令,文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S
13、6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K
14、8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S
15、6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K
16、8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S
17、6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K
18、8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3再令20821,3135aaa
19、nm可得.(2)当*Nn时,由已知(以mn代替2)可得82121232nnnaaa.于是8)(1212112112nnnnaaaa,即6,81211aabbbnn.所以nb是以 6 为首项,8 为公差的等差数列.(3)由(1)(2)解答可知28,281212naanbnnn即.另由已知(令1m)可得211212naaann.那么nnnnaaaannnn21222812212121,于是12nnnqc.当1q时,12642nnnSn;当1q时,12102642nnnqqqqS.两边同乘以q,可得nnnqqqqqS2642321.上述两式相减得qnqqnnqqqnqqqqSqnnnnnnn1112
20、21122121112.所以21)1(112qnqqnSnnn.综上所述,1,11,111221qnnqqnqqnSnnn文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R
21、3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE
22、8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R
23、3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE
24、8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R
25、3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE8X3S8X8J9 ZX5Z8K8S1R3文档编码:CA4G8C1G4S6 HE
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