问题 解答题
在数列{an}中,a1=-
1
3
,n∈N*
,当n≥2时,有3an-2an-1+n+2=0,设bn=an+n+1.
(I)求b1,b2
(II)证明数列{bn-1}是等比数列;
(III)设cn=
(
2
3
)
n
2
b2n
+bn
,求数列{cn}的前n项和Tn
答案

(I)∵a1=-

1
3
,bn=an+n+1∴b1=a1+2=
5
3

当n=2时,3a2-2a1+4=0可得a2=-

14
9

b2=3+a2=

13
9

(II)由3an-2an-1+n+2=0得,3(an+n)=2(an-1+n-1)

an+n
an-1+n-1
=
2
3
,n≥2即
bn-1
bn-1-1
=
2
3

b1- 1=

2
3
≠0

{bn-1}是以

2
3
为首项,
2
3
为公比的等比数列

(III)由(I)可得bn=

2
3
bn-1+
1
3

∴2bn-1+1=3bn,所以bn=1+(

2
3
)n

cn=

(
2
3
)
n
2
b2n
+bn
=
(
2
3
)
n
(2bn+1)bn
=
bn-bn+1
bnbn+1
=
1
bn+1
-
1
bn

Tn=C1+C2+…+Cn=

1
bn+1
-
1
b1
=
3n+1
2n+13n+1
-
3
5

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