已知数列{log2(an-1)}(n∈N*)为等差数列,且a1=3,a2=5,则limn→∞(1a2?a1+1a3?a2+…+1an+1?an)=___

2025-05-14 13:08:16
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回答(1):

数列{log2(an-1)}(n∈N*)为等差数列,且a1=3,a2=5
数列的公差为log24-log22=1,
故log2(an-1)=1+(n-1)×1=n,即an-1=2n,an=1+2n
∴an+1-an=2n+1+1-2n-1=2n

lim
n→∞
(
1
a2?a1
+
1
a3?a2
+…+
1
an+1?an
)=
lim
n→∞
(
1
2
+
1
22
+…+
1
2n
)=
lim
n→∞
(
1
2
×(1?
1
2n
)
1?
1
2
)=
lim
n→∞
(1?
1
2n
)=1

故答案为1