已知数列{an}是公差不为0的等差数列,a3=6,且a1,a2,a4成等比数列,数列{bn}满足bn+1=2bn+1,n∈N*,且

2025-05-14 08:34:14
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回答(1):

解答:(1)解:设公差为d≠0,
∵a3=6,且a1,a2,a4成等比数列,
∴a1+2d=6,且(a1+d)2=a1?(a1+3d),
解得a1=2,d=2.
∴数列{an}的通项公式为an=2+(n-1)×2=2n;
∵bn+1=2bn+1,
∴bn+1+1=2(bn+1),
∵b1=3,
∴数列{bn+1}是以4为首项,2为公比的等比数列,
∴bn+1=2n+1
∴bn=2n+1-1;
(2)证明:cn=

1
an?log2(bn+1)
=
1
2n(n+1)
=
1
2
1
n
-
1
n+1
),
∴Sn=
1
2
(1-
1
2
+
1
2
-
1
3
+…+
1
n
-
1
n+1
)=
1
2
(1-
1
n+1
)<
1
2

∴Sn
1
2