问题
解答题
数列{an}的各项均为正数,Sn为其前n项和,对于任意n∈N*,总有an,Sn,an2成等差数列. (1)求数列{an}的通项公式; (2)设数列{bn}的前n项和为Tn,且bn=
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答案
(1)根据题意,对于任意n∈N*,总有an,Sn,an2成等差数列,则对于n∈N*,总有2Sn=an+an2①成立
∴2Sn-1=an-1+an-1 2(n≥2)②
①-②得2an=an+an2-an-1-an-12,即an+an-1=(an+an-1)(an-an-1);
∵an,an-1均为正数,
∴an-an-1=1(n≥2)
∴数列{an}是公差为1的等差数列,
又n=1时,2S1=a1+a12,解得a1=1
∴an=n.(n∈N*)
(2)证明:由(1)的结论,an=n;对任意实数x∈(1,e],有0<lnx<1,
对于任意正整数n,总有bn=
≤lnnx an2
.1 n2
∴Tn≤
+1 12
+…+1 22
<1+1 n2
+1 1•2
+…+1 2•3 1 (n-1)n
=1+1-
+1 2
-1 2
+…+1 3
-1 n-1
=2-1 n
<21 n
对任意实数x∈(1,e](e是常数,e=2.71828…)和任意正整数n,总有Tn<2