问题 解答题
设数列{an}的前n项和为Sn,a1=1,Sn+1=4an+2(n∈N+
(1)若bn=an+1-2an,求bn
(2)若cn=
1
an+1-2an
,求{cn}的前6项和T6
(3)若dn=
an
2n
,证明{dn}是等差数列.
答案

解(1)∵a1=1,Sn+1=4an+2(n∈N+),

∴Sn+2=4an+1+2an+2=Sn+2-Sn+1=4(an+1-an),

∴an+2-2an+1=2(an+1-2an

即bn+1=2bn

∴{bn}是公比为2的等比数列,且b1=a2-2a1

∵a1=1,a2+a1=S2

即a2+a1=4a1+2,

∴a2=3a1+2=5,

∴b1=5-2=3,

bn=3•2n-1

(2)∵cn=

1
an+1-2an
=
1
bn
=
1
3•2n-1

c1=

1
3•21-1
=
1
3
,∴cn=
1
3
•(
1
2
)n-1

∴{cn}是首项为

1
3
,公比为
1
2
的等比数列.

T6=

1
3
[1-(
1
2
)
6
]
1-
1
2
=
2
3
(1-
1
64
)=
61
96

(3)∵dn=

an
2n
bn=3•2n-1

dn+1-dn=

an+1
2n+1
-
an
2n
=
an+1-2an
2n+1
=
bn
2n+1

dn+1-dn=

3•2n-1
2n+1
=
3
4

∴{dn}是等差数列.

单项选择题
单项选择题