问题
解答题
已知数列{an}的前n项和为Sn,满足Sn=2an-n
(1)求数列{an}的通项公式;
(2)设bn=(2n+1)(an+1),求数列{bn}的前n项和Tn.
答案
(1)∵Sn=2an-n
当n=1时,a1=S1=2a1-1,∴a1=1
当n≥2时,Sn=2an-n ①
Sn-1=2an-1-n+1 ②
①-②得an=2an-1+1即an+1=2(an-1+1)
∵a1+1=2≠0∴an-1+1≠0
∴
=2an+1 an-1+1
∴{an+1}是以首项为2,公比为2的等比数列
an+1=2•2n-1=2n
∴an=2n-1
(2)bn=(2n+1)•2n
Tn=3•2+5•22+7•23+…+(2n-1)•2n-1+(2n+1)•2n,
2Tn=3•22+5•23+7•24+…+(2n-1)•2n+(2n+1)•2n+1,
∴-Tn=6+2(22+23+24+…+2n)-(2n+1)•2n+1,
∴Tn=2+(2n-1)•2n+1.