问题
解答题
已知数列{an},{bn}是各项均为正数的等比数列,设cn=
(Ⅰ)证明:数列{cn}是等比数列,数列{lnan}是等差数列. (Ⅱ)设数列{lnan},{lnbn}的前n项和分别是Sn,Tn.若a1=2,
(Ⅲ)在(Ⅱ)条件下,设dn=
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答案
(1)设数列{an}、bn的公比分别为p、q(p>0,q>0),
则由题意可得
=p,an an-1
=q,bn bn-1
∴
=cn cn-1
= pq,c1=a1•b1an•bn an-1•bn-1
所以数列cn以a1•b1为首项,以pq为公比的等比数列
又因为lnan-lnan-1=ln
=lnp,an an-1
数列lnan以lna1为首项,以lnp为公差的等差数列
(2)由题意可得sn=n•ln2+
×lnp,Tn=n•lnb1+n(n-1) 2
×lnqn(n-1) 2
∴
=Sn Tn
=n•ln2+
• lnpn(n-1) 2 n•lnb1+
•lnqn(n-1) 2
=2ln2+(n-1)•lnp 2lnb1+(n-1)•lnq n 2n+1
∴
=n•lnp+(ln4-lnp) n•lnq+(2lnb1-lnq)
=n+ ln4-lnp lnp n•
+lnq lnp 2lnb1-lnq lnp n 2n+1
∴
=0,ln4-lnp lnp
=2,lnq lnp
=12lnb1-lnq lnp
∴p=4,q=16,b1=8
∴an=2•4n-1=22n-1,bn=8•16n-1=24n-1
(III)由(II)可得dn=6•cn bn+1-4an+1-4an+2
=6•4n 8•16n-8•4n-2•4n+2
=3•4n 4•(4n)2-5•4n+1
=
=3•4n (4n-1)(4n+1-1)
-1 4n-1 1 4n+1-1
∴d1+d2+d3+…+dn
=
-1 41-1
+1 42-1
-1 42-1
+…+1 43-1
-1 4n-1 1 4n+1-1
=
-1 3 1 4n+1-1