设等差数列{an}的前n项和为Sn,等比数列{bn}的前n项和为Tn,已知bn>0(n∈N*),a1=b1=1,a2+b

问题描述:

设等差数列{an}的前n项和为Sn,等比数列{bn}的前n项和为Tn,已知bn>0(n∈N*),a1=b1=1,a2+b3=a3,S5=5(T3+b2).
(Ⅰ)求数列{an},{bn}的通项公式;
(Ⅱ)求和:
b
1个回答 分类:数学 2014-11-27

问题解答:

我来补答
(Ⅰ)设{an}的公差为d,数列{bn}的公比为q,则
∵a1=b1=1,a2+b3=a3,S5=5(T3+b2),
∴q2=d,1+2d=1+2q+q2
∴q2-2q=0,
∵q≠0,∴q=2,∴d=4
∴an=4n-3,bn=2n-1
(Ⅱ)∵
bn
TnTn+1=
bn+1
qTnTn+1=
1
q(
1
Tn−
1
Tn+1)

b1
T1T2+
b2
T2T3+…+
bn
TnTn+1=
1
q(
1
T1−
1
T2+
1
T2−
1
T3+…+
1
Tn−
1
Tn+1)
=
1
q(
1
T1−
1
Tn+1)=
1
q(1-
2
2n+1−1).
 
 
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