已知数列{an}的前n项和为Sn,且满足Sn=2an-n(n∈N+)(I)求证{an+1}是等比数列,并求an;(II)bn=nan+

2025-03-04 08:12:46
推荐回答(1个)
回答1:

(I)∵Sn=2an-n(n∈N*),
∴当n≥2时,Sn-1=2an-1-(n-1).
两式相减得an=2an-2an-1-1,即an=2an-1+1(n≥2).…(3分)
又∵a1=1,可知an>0,
∴当n≥2时,
an+1
an?1+1
=2

∴{an+1}是首项为2,公比为2的等比数列,
故an+1=2?2n-1=2n,也即an=2n-1
(II)bn=nan+n=n?2n
Tn=1?2+2?22+3?23+…+(n-1)?2n-1+n?2n
2Tn=1?22+2?23+3?24+…+(n-1)?2n+n?2n+1
两式相减,得-Tn=2+22+23+…+2n-n?2n+1
-Tn=
2(1?2n)
1?2
-n?2n+1
得Tn=(n-1)?2n+1+2