已知各项均为正数的数列{an}的前n项和Sn满足6Sn=(an+1)(an+2),n∈N+,且a

已知各项均为正数的数列{an}的前n项和Sn满足6Sn=(an+1)(an+2),n∈N+,且a
已知各项均为正数的数列{an}的前n项和Sn满足6Sn=(an+1)(an+2),n∈N+,
且a2 a4 a9成等比数列
(Ⅰ)求{an}的通项公式;
Ⅱ 设bn=-1的n+1次方乘以an乘以a(n+1) Tn为数列bn的前n项和 求T2n
imtasa 1年前 已收到1个回答 举报

sunkist1976 幼苗

共回答了19个问题采纳率:94.7% 举报

n=1时,6a1=a12+3a1+2,且a1>1,解得a1=2.
n≥2时,6Sn=an2+3an+2,6Sn-1=an-12+3an-1+2,两式相减得(an+an-1)(an-an-1-3)=0,
∵an+an-1>0,
∴an-an-1=3,
∴{an}为等差数列,
∵a1=2,
∴an=3n-1.
(II)证明:∵数列{bn}满足an(2bn−1)=1,
∴bn=log2
3n
3n−1
∴Tn=b1+b2+…+bn=log2(
3
2
×
6
5
×…×
3n
3n−1
)
要证2Tn+1<log2(an+3),即证2log2(
3
2
×
6
5
×…×
3n
3n−1
)+1<log2(an+3)
即证(
3
2
×
6
5
×…×
3n
3n−1
)2<
3n+2
2
即证
2(
3
2
×
6
5
×…×
3n
3n−1
)2
3n+2
<1
令cn=
2(
3
2
×
6
5
×…×
3n
3n−1
)2
3n+2
,

cn+1
cn

9n2+18n+9
9n2+21n+10
<1
∵cn>0,∴cn+1<cn,
∴{cn}是单调递减数列
∴cn≤c1=
2×(
3
2
)2
3×1+2

9
10
<1
∴cn=
2(
3
2
×
6
5
×…×
3n
3n−1
)2
3n+2
<1
故2Tn+1<log2(an+3).

1年前

2
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