问题 解答题
设数列{an} 对任意n∈N*和实数常数,有
an-2an+1
anan+1
=t-2
,t∈R,a1=
1
3

(1)若{
1-an
an
}是等比数列,求{an} 的通项公式;
(2)设{bn}满足bn=(1-an)an,其前n项和Tn,求证:Tn>
2
3
2n-1
2n+1+1
答案

(1)由

an-2an+1
anan+1
=t-2,t∈R,a1=
1
3

1
an+1
-1=2(
1
an
-1) +t•
1
a1
-1=2,

∵{

1-an
an
}是等比数列,

1
an
-1=2n

an=

1
2n+1

(2)由bn=(1-an)anbn=(1-

1
2n+1
) •
1
2n+1
=
2n
(2n+1)2
1
2n+1
-
1
2n+1+1

前n项和Tn=b1+b2+…+bn

1
3
-
1
2n+1+1

=

2
3
2n-1
2n+1+1

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