问题
解答题
设数列{an} 对任意n∈N*和实数常数,有
(1)若{
(2)设{bn}满足bn=(1-an)an,其前n项和Tn,求证:Tn>
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答案
(1)由
=t-2,t∈R,a1=an-2an+1 anan+1
,1 3
得
-1=2(1 an+1
-1) +t•1 an
-1=2,1 a1
∵{
}是等比数列,1-an an
∴
-1=2n,1 an
得an=
.1 2n+1
(2)由bn=(1-an)an得bn=(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