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
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(1)由n∈N*,
-1=4an(an+1),得(an+1+2an+1)(an+1-2an-1)=0a 2n+1
∵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•2n①
则2Tn′=1•22+2•23+3•24+…+n•2n+1②
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
≥21 y
,1 xy
∴(x+y)(
+1 x
)≥4∴1 y
+1 x
≥1 y
当且仅当x=y时等号成立.4 x+y
∴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(1-2(k-1) k+1
)>2(1-2 k+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+1 8
+83 140
>1+1 8
=1 2 3 2