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→∞
=3,则an bn
lim n→∞
= ___ .
b1+b2+…bn
na4n
答
设{an}、{bn}的公差分别为d1 和d2,
则由
lim n→∞
=an bn
lim n→∞
=3,∴
a1+(n-1)d1
b1+(n-1)d2
=3,d1=3d2.d1 d2
∴
lim n→∞
=
b1+b2+…bn
na4n
lim n→∞
=nb1+
•d2 n(n-1) 2 n[a1 +(4n-1)•3d2]
lim n→∞
b1+
•d2
n-1 2
a1+(4n-1)•3d2
═
lim n→∞
=
+b1 n-1
d2 2
+(a1 n-1
) •3d2
4n-1 n-1
=
d2
1 2 12d2
.1 24