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

2025-03-22 06:24:52
推荐回答(1个)
回答(1):

(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
2
Tn
=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