问题 解答题
已知数列{an}的各项均为正值,a1=1,对任意n∈N*,an+12-1=4an(an+1),bn=log2(an+1)都成立.
(1)求数列{an}、{bn}的通项公式;
(2)令cn=an•bn,求数列{cn}的前n项和Tn
(3)当k>7且k∈N*时,证明对任意n∈N*,都有
1
bn
+
1
bn+1
+
1
bn+2
+…+
1
bnk-1
3
2
成立.
答案

(1)由n∈N*

a2n+1
-1=4an(an+1),得(an+1+2an+1)(an+1-2an-1)=0

∵数列{an}的各项均为正值,an+1+2an+1>0,∴an+1=2an+1,整理为an+1+1=2(an+1)

又a1+1=2≠0∴数列{an+1}为等比数列,

an+1=(a1+1)•2n-1=2n∴数列{an}的通项公式an=2n-1

数列{bn}的通项公式bn=log2(2n-1+1)=n

(2)由(1)cn=an•bn=n•(2n-1)

所以Tn=1•21+2•22+3•23+…+n•2n-(1+2+3+…+n)

令Tn′=1•21+2•22+3•23+…+n•2n

则2Tn′=1•22+2•23+3•24+…+n•2n+1

①-②得-Tn′=1•21+22+23+24+…++2n-n•2n+1=2n+1-2-n•2n+1=(1-n)2n+1-2

(3)法1:设S=

1
bn
+
1
bn+1
+
1
bn+2
+…+
1
bnk-1
=
1
n
+
1
n+1
+
1
n+2
+…+
1
nk-1

2S=(

1
n
+
1
nk-1
)+(
1
n+1
+
1
nk-2
)+(
1
n+2
+
1
nk-3
)+…+(
1
nk-1
+
1
n
)

当x>0,y>0时,x+y≥2

xy
1
x
+
1
y
≥2
1
xy

(x+y)(

1
x
+
1
y
)≥4∴
1
x
+
1
y
4
x+y
当且仅当x=y时等号成立.

∴上述(1)式中,k>7,n>0,n+1,n+2,…,nk-1全为正,

2S>

4
n+nk-1
+
4
n+1+nk-2
+
4
n+2+nk-3
+…+
4
nk-1+n
=
4n(k-1)
n+nk-1

S>

2(k-1)
1+k-
1
n
2(k-1)
k+1
=2(1-
2
k+1
)>2(1-
2
7+1
)=
3
2

法2∵k≥8,S≥

1
n
+
1
n+1
+…+
1
8n-1

=

1
n
+…+
1
2n-1
+
1
2n
+…+
1
3n-1
+
1
3n
+…+
1
4n-1
+…+
1
8n-1
1
2n-1
+…+
1
2n-1
+
1
3n-1
+…+
1
3n-1
+
1
4n-1
+…+
1
4n-1
+…+
1
8n-1
+…+
1
8n-1

=

n
2n-1
+
n
3n-1
+
n
4n-1
+…+
n
8n-1
1
2
+
1
3
+
1
4
+
1
5
+
1
6
+
1
7
+
1
8

=1+

1
4
+
1
5
+
1
7
+
1
8
=1+
83
140
+
1
8
>1+
1
2
=
3
2

单项选择题
单项选择题