已知正项数列{an}的前n项和为Sn,且2Sn=an2+an,数列{bn}满足b1=1,2bn-bn-1=0(n≥2,n

已知正项数列{an}的前n项和为Sn,且2Sn=an2+an,数列{bn}满足b1=1,2bn-bn-1=0(n≥2,n∈N *)(Ⅰ)求
已知正项数列{an}的前n项和为Sn,且2Sn=an2+an,数列{bn}满足b1=1,2bn-bn-1=0(n≥2,n∈N *
(Ⅰ)求数列{an}的通项公式;
(Ⅱ)若cn=anbn,求数列{cn}的前n项和Tn
chenpuyou 1年前 已收到1个回答 举报

瑶小若 幼苗

共回答了17个问题采纳率:88.2% 举报

(1)n=1时,2S1=2a1=a12+a1
a12-a1=0,解得a1=0(各项均为正数,舍去)或a1=1,
n≥2时,
2Sn=an2+an
2Sn-1=an-12+an-1
2Sn-2Sn-1=2an=an2+an-an-12-an-1
an2-an-12-an-an-1=0
(an+an-1)(an-an-1)-(an+an-1)=0
(an+an-1)(an-an-1-1)=0
∵数列各项均为正,∴an-an-1=1,
∴数列{an}是以1为首项,1为公差的等差数列.
∴an=1+n-1=n.
(2)∵数列{bn}满足b1=1,2bn-bn-1=0(n≥2,n∈N *),
∴{bn}是首项为1,公比为[1/2]的等比数列,
∴bn=(
1
2)n?1.
∴cn=anbn=n?(
1
2)n?1,
∴Tn=1+2×[1/2]+3×(
1
2)2+…+n?(
1
2)n?1,①
[1/2]Tn=[1/2+2×(
1
2)2+3×(
1
2)3+…+n?(
1
2)n,②
①-②,得:
1
2Tn=1+
1
2+(
1
2)2+(
1
2)3+…+(
1
2)n?1?n?(
1
2)n
=
1?(
1
2)n
1?
1
2]-n?(
1
2)n
=2-(n+2)?([1/2])n
∴Tn=4?(2n+4)?(
1
2)n.

1年前

4
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