问题 解答题
已知数列{an}的前n项和为Sna1=
1
4
,且2Sn=2Sn-1+2an-1+1(n≥2,n∈N*).数列{bn}满足b1=
3
4
,且3bn-bn-1=n(n≥2,n∈N*).
(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
答案

(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
=
1
2
n-
1
4
.(5分)

又3bn-bn-1=n(n≥2),

bn=

1
3
bn-1+
1
3
n(n≥2),

bn-an=

1
3
bn-1+
1
3
n-
1
2
n+
1
4
=
1
3
bn-1-
1
6
n+
1
4

=

1
3
(bn-1-
1
2
n+
3
4
)

=

1
3
[bn-1-
1
2
(n-1)+
1
4
]

=

1
3
(bn-1-an-1).(8分)

b1-a1=

1
2
≠0,

∴对n∈N*,bn-an≠0,得

bn-an
bn-1-an-1
=
1
3
  (n≥2).

∴数列{bn-an}是首项为

1
2
公比为
1
3
等比数列.(9分)

(Ⅲ)由(Ⅱ)得bn-an=

1
2
•(
1
3
)n-1

b n=

n
2
-
1
4
+
1
2
•(
1
3
)n-1 (n∈N*).(11分)

b1-a1+b2-a2++bn-an=

1
2
[1-(
1
3
)
n
]
1-
1
3

b1+b2++bn-(a1+a2++an)=

3
4
[1-(
1
3
)n],

Tn-

n2
4
=
3
4
[1-(
1
3
)n].

Tn=

n2
4
+
3
4
[1-(
1
3
)n]  (n∈N*).(14分)

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