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2022年高考数学天津18

  2022-12-16 20:38:14  

(15分)设{an}是等差数列,{bn}是等比数列,且a1=b1=a2b2=a3b3=1
(1)求{an}{bn}的通项公式;
(2)设{an}的前n项和为Sn,求证:(Sn+1+an+1)bn=Sn+1bn+1Snbn
(3)求2nk=1[ak+1(1)kak]bk
分析:(1)设等差数列{an}的公差为d,等比数列{bn}的公比为q,由a1=b1=a2b2=a3b3=1,可得1+dq=11+2dq2=1,解得dq,即可得出an
(2)由等比数列的性质及通项公式与前n项和的关系结合分析法能证明(Sn+1+an+1)bn=Sn+1bn+1Snbn
(3)先求出[a2k(1)2k1a2k1]b2k1+[a2k+1(1)2k2k]b2k=2k4k,利用并项求和,结合错位相减法能求出结果.
解:(1)设等差数列{an}的公差为d,等比数列{bn}的公比为q
a1=b1=a2b2=a3b3=1
1+dq=11+2dq2=1
解得d=q=2
an=1+2(n1)=2n1bn=2n1
(2)证明:bn+1=2bn0
要证明(Sn+1+an+1)bn=Sn+1bn+1Snbn
即证明(Sn+1+an+1)bn=2Sn+1bnSnbn
即证明Sn+1+an+1=2Sn+1Sn
即证明an+1=Sn+1Sn
由数列的通项公式和前n项和的关系得:an+1=Sn+1Sn
(Sn+1+an+1)bn=Sn+1bn+1Snbn
(3)[a2k(1)2k1a2k1]b2k1+[a2k+1(1)2k2k]b2k
=(4k1+4k3)×22k2+[4k+1(4k1)]×22k1=2k4k
2nk=1[ak+1(1)kak]bk=nk=1[a2k(1)2k1a2k1]b2k1+[a2k+1(1)2ka2k]b2k
=nk=12k4k
Tn=nk=12k4k
Tn=2×4+4×42+6×43++2n×4n,①
4Tn=2×42+4×43+6×44++2n×4n+1,②
②,得:
3Tn=2(4+42+43+44++4n)2n4n+1
=2×4(14n)142n×4n+1
=(26n)4n+183
Tn=(6n2)4n+1+89
2nk=1[ak+1(1)kak]bk=(6n2)4n+1+89
点评:本题考查了等差数列与等比数列的通项公式与求和公式、错位相减法,考查了推理能力与计算能力,属于难题.
 

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