已知数列{an}的前n项和为Sn,a1=
(Ⅰ)求证:数列{an}为等差数列; (Ⅱ)求证:数列{bn-an}为等比数列; (Ⅲ)求数列{bn}的通项公式以及前n项和Tn. |
(Ⅰ)证明:∵2Sn=2Sn-1+2an-1+1(n≥2,n∈N*),
∴当n≥2时,2an=2an-1+1,
可得an-an-1=
.1 2
∴数列{an}为等差数列.(4分)
(Ⅱ)证明:∵{an}为等差数列,公差d=
,1 2
∴an=a1+(n-1)×
=1 2
n-1 2
.(5分)1 4
又3bn-bn-1=n(n≥2),
∴bn=
bn-1+1 3
n(n≥2),1 3
∴bn-an=
bn-1+1 3
n-1 3
n+1 2
=1 4
bn-1-1 3
n+1 6 1 4
=
(bn-1-1 3
n+1 2
)3 4
=
[bn-1-1 3
(n-1)+1 2
]1 4
=
(bn-1-an-1).(8分)1 3
又b1-a1=
≠0,1 2
∴对n∈N*,bn-an≠0,得
=bn-an bn-1-an-1
(n≥2).1 3
∴数列{bn-an}是首项为
公比为1 2
等比数列.(9分)1 3
(Ⅲ)由(Ⅱ)得bn-an=
•(1 2
)n-1,1 3
∴b n=
-n 2
+1 4
•(1 2
)n-1 (n∈N*).(11分)1 3
∵b1-a1+b2-a2++bn-an=
,
[1-(1 2
)n]1 3 1- 1 3
∴b1+b2++bn-(a1+a2++an)=
[1-(3 4
)n],1 3
∴Tn-
=n2 4
[1-(3 4
)n].1 3
∴Tn=
+n2 4
[1-(3 4
)n] (n∈N*).(14分)1 3