1数列经典题目集锦一一、构造法证明等差、等比类型一:按已有目标构造1、数列{an},{bn},{cn}满足:bn=an-2an+1,cn=an+1+2an+2-2,n∈N*.(1)若数列{an}是等差数列,求证:数列{bn}是等差数列;(2)若数列{bn},{cn}都是等差数列,求证:数列{an}从第二项起为等差数列;(3)若数列{bn}是等差数列,试判断当b1+a3=0时,数列{an}是否成等差数列?证明你的结论.类型二:整体构造2、设各项均为正数的数列{an}的前n项和为Sn,已知a1=1,且(Sn+1+λ)an=(Sn+1)an+1对一切n∈N*都成立.(1)若λ=1,求数列{an}的通项公式;(2)求λ的值,使数列{an}是等差数列.二、两次作差法证明等差数列3、设数列na的前n项和为nS,已知11,6,1321aaa,且*1,)25()85(NnBAnSnSnnn,(其中A,B为常数).(1)求A与B的值;(2)求数列na为通项公式;三、数列的单调性4.已知常数0,设各项均为正数的数列na的前n项和为nS,满足:11a,11131nnnnnnaSSaa(*nN).(1)若0,求数列na的通项公式;(2)若112nnaa对一切*nN恒成立,求实数的取值范围.5.设数列na是各项均为正数的等比数列,其前n项和为nS,若1564aa,5348SS.(1)求数列na的通项公式;(2)对于正整数,,kml(kml),求证:“1mk且3lk”是“5,,kmlaaa这三项经适当排序后能构成等差数列”成立的充要条件;(3)设数列nb满足:对任意的正整数n,都有121321nnnnabababab13246nn,且集合*|,nnbMnnNa中有且仅有3个元素,求的取值范围.2四、隔项(分段)数列问题6.已知数列{an}中,a1=1,an+1=13an+n(n为奇数),an-3n(n为偶数).(1)是否存在实数λ,使数列{a2n-λ}是等比数列?若存在,求出λ的值;若不存在,请说明理由;(2)若Sn是数列{an}的前n项的和,求满足Sn>0的所有正整数n.7.若nb满足:对于Nn,都有2nnbbd(d为常数),则称数列nb是公差为d的“隔项等差”数列.(Ⅰ)若17,321cc,nc是公差为8的“隔项等差”数列,求nc的前15项之和;(Ⅱ)设数列na满足:1aa,对于Nn,都有12nnaan.①求证:数列na为“隔项等差”数列,并求其通项公式;②设数列na的前n项和为nS,试研究:是否存在实数a,使得22122kkkSSS、、成等比数列(*Nk)?若存在,请求出a的值;若不存在,请说明理由.五、数阵问题8.已知等差数列{an}、等比数列{bn}满足a1+a2=a3,b1b2=b3,且a3,a2+b1,a1+b2成等差数列,a1,a2,b2成等比数列.(1)求数列{an}和数列{bn}的通项公式;(2)按如下方法从数列{an}和数列{bn}中取项:第1次从数列{an}中取a1,第2次从数列{bn}中取b1,b2,第3次从数列{an}中取a2,a3,a4,第4次从数列{bn}中取b3,b4,b5,b6,……第2n-1次从数列{an}中继续依次取2n-1个项,第2n次从数列{bn}中继续依次取2n个项,……由此构造数列{cn}:a1,b1,b2,a2,a3,a4,b3,b4,b5,b6,a5,a6,a7,a8,a9,b7,b8,b9,b10,b11,b12,…,记数列{cn}的前n项和为Sn.求满足Sn22014的最大正整数n.3数列经典题目集锦答案1.证明:(1)设数列{an}的公差为d,∵bn=an-2an+1,∴bn+1-bn=(an+1-2an+2)-(an-2an+1)=(an+1-an)-2(an+2-an+1)=d-2d=-d,∴数列{bn}是公差为-d的等差数列.(4分)(2)当n≥2时,cn-1=an+2an+1-2,∵bn=an-2an+1,∴an=bn+cn-12+1,∴an+1=bn+1+cn2+1,∴an+1-an=bn+1+cn2-bn+cn-12=bn+1-bn2+cn-cn-12.∵数列{bn},{cn}都是等差数列,∴bn+1-bn2+cn-cn-12为常数,∴数列{an}从第二项起为等差数列.(10分)(3)结论:数列{an}成等差数列.证明如下:(证法1)设数列{bn}的公差为d′,∵bn=an-2an+1,∴2nbn=2nan-2n+1an+1,∴2n-1bn-1=2n-1an-1-2nan,…,2b1=2a1-22a2,∴2nbn+2n-1bn-1+…+2b1=2a1-2n+1an+1,设Tn=2b1+22b2+…+2n-1bn-1+2nbn,∴2Tn=22b1+…+2nbn-1+2n+1bn,两式相减得:-Tn=2b1+(22+…+2n-1+2n)d′-2n+1bn,即Tn=-2b1-4(2n-1-1)d′+2n+1bn,∴-2b1-4(2n-1-1)d′+2n+1bn=2a1-2n+1an+1,∴2n+1an+1=2a1+2b1+4(2n-1-1)d′-2n+1bn=2a1+2b1-4d′-2n+1(bn-d′),∴an+1=2a1+2b1-4d′2n+1-(bn-d′).(12分)令n=2,得a3=2a1+2b1-4d′23-(b2-d′)=2a1+2b1-4d′23-b1,∵b1+a3=0,∴2a1+2b1-4d′23=b1+a3=0,∴2a1+2b1-4d′=0,∴an+1=-(bn-d′),∴an+2-an+1=-(bn+1-d′)+(bn-d′)=-d′,∴数列{an}(n≥2)是公差为-d′的等差数列.(14分)∵bn=an-2an+1,令n=1,a1-2a2=-a3,即a1-2a2+a3=0,∴数列{an}是公差为-d′的等差数列.(16分)(证法2)∵bn=an-2an+1,b1+a3=0,令n=1,a1-2a2=-a3,即a1-2a2+a3=0,(12分)∴bn+1=an+1-2an+2,bn+2=an+2-2an+3,∴2bn+1-bn-bn+2=(2an+1-an-an+2)-2(2an+2-an+1-an+3).∵数列{bn}是等差数列,∴2bn+1-bn-bn+2=0,∴2an+1-an-an+2=2(2an+2-an+1-an+3).(14分)∵a1-2a2+a3=0,∴2an+1-an-an+2=0,∴数列{an}是等差数列.(16分)2.解析:(1)若λ=1,则(Sn+1+1)an=(Sn+1)an+1,a1=S1=1.4∵an>0,Sn>0,∴Sn+1+1Sn+1=an+1an,(2分)∴S2+1S1+1·S3+1S2+1·…·Sn+1+1Sn+1=a2a1·a3a2·…·an+1an,化简,得Sn+1+1=2an+1.①(4分)∴当n≥2时,Sn+1=2an.②①-②,得an+1=2an,∴an+1an=2(n≥2).(6分)∵当n=1时,a2=2,∴n=1时上式也成立,∴数列{an}是首项为1,公比为2的等比数列,an=2n-1(n∈N*).(8分)(2)令n=1,得a2=λ+1.令n=2,得a3=(λ+1)2.(10分)要使数列{an}是等差数列,必须有2a2=a1+a3,解得λ=0.(11分)当λ=0时,Sn+1an=(Sn+1)an+1,且a2=a1=1.当n≥2时,Sn+1(Sn-Sn-1)=(Sn+1)(Sn+1-Sn),整理,得S2n+Sn=Sn+1Sn-1+Sn+1,Sn+1Sn-1+1=Sn+1Sn,(13分)从而S2+1S1+1·S3+1S2+1·…·Sn+1Sn-1+1=S3S2·S4S3·…·Sn+1Sn,化简,得Sn+1=Sn+1,∴an+1=1.(15分)综上所述,an=1(n∈N*),∴λ=0时,数列{an}是等差数列.(16分)3.解析:(1)由11,6,1321aaa,得18,7,1321SSS.把2,1n分别代入*1,)25()85(NnBAnSnSnnn,得48228BABA,解得,8,20BA.(2)由(1)知,82028)(511nSSSSnnnnn,即82028511nSSnannn,①又8)1(2028)1(5122nSSannnn.②②-①得,20285)1(51212nnnnaanaan,即20)25()35(12nnanan.③又20)75()25(23nnanan.④④-③得,0)2)(25(123nnnaaan,520n,∴02123nnnaaa,又32215aaaa,所以32120aaa,因此,数列na是首项为1,公差为5的等差数列.故45)1(51nnan.4.解析:(1)0时,111nnnnnaSSaa5∴1nnnnaSSa∵0na,∴0nS∴1nnaa,∵11a,∴1na(2)∵11131nnnnnnaSSaa0na,∴1131nnnnnSSaa则212131SSaa,2323231SSaa,,11131nnnnnSSaa2n相加,得2113331nnnSna则3322nnnSnan,该式对1n也成立,∴*332nnnSnanN.③∴1*13312nnnSnanN.④④-③,得1113333122nnnnnanana即11333322nnnnnana∵0,∴133330,022nnnn.∵112nnaa对一切*nN恒成立,∴332nn1133()22nn对一切*nN恒成立.即233nn对一切*nN恒成立.记233nnnb,则111423622233333333nnnnnnnnnnbb当1n时,10nnbb;当2n时,10nnbb∴1213bb是{}nb中的最大项.6综上所述,的取值范围是13.5.解析:(1)数列na是各项均为正数的等比数列,215364aaa,38a,又5348SS,2458848aaqq,2q,3822nnna;……4分(2)(ⅰ)必要性:设5,,kmlaaa这三项经适当排序后能构成等差数列,①若25kmlaaa,则10222kml,1022mklk,11522mklk,1121,24mklk13mklk.…………6分②若25mklaaa,则22522mkl,1225mklk,左边为偶数,等式不成立,③若25lkmaaa,同理也不成立,综合①②③,得1,3mklk,所以必要性成立.…………8分(ⅱ)充分性:设1mk,3lk,则5,,kmlaaa这三项为135,,kkkaaa,即5,2,8kkkaaa,调整顺序后易知2,5,8kkkaaa成等差数列,所以充分性也成立.综合(ⅰ)(ⅱ),原命题成立.…………10分(3)因为11213213246nnnnnababababn,即123112122223246nnnnnbbb