等差数列{an}、{bn}的公差都不为零,若limn→∞anbn=3,则limn→∞b1+b2+…bnna4n= _ .

问题描述:

等差数列{an}、{bn}的公差都不为零,若

lim
n→∞
an
bn
=3,则
lim
n→∞
b1+b2+…bn
na4n
= ___ .

设{an}、{bn}的公差分别为d1 和d2
则由

lim
n→∞
an
bn
=
lim
n→∞
a1+(n-1)d1
b1+(n-1)d2
=3,∴
d1
d2
=3,d1=3d2
lim
n→∞
b1+b2+…bn
na4n
=
lim
n→∞
nb1+
n(n-1)
2
d2 
n[a1 +(4n-1)•3d2]
=
lim
n→∞
b1+
n-1
2
d2
a1+(4n-1)•3d2
 
lim
n→∞
b1
n-1
 +
d2
2
 
a1
n-1
+(
4n-1
n-1
•3d2
=
1
2
d2
12d2
=
1
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