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=
(3)若dn=
|
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
,∴cn=1 3
•(1 3
)n-11 2
∴{cn}是首项为
,公比为1 3
的等比数列.1 2
∴T6=
=
[1-(1 3
)6]1 2 1- 1 2
(1-2 3
)=1 64
.61 96
(3)∵dn=
,bn=3•2n-1,an 2n
∴dn+1-dn=
-an+1 2n+1
=an 2n
=an+1-2an 2n+1 bn 2n+1
即dn+1-dn=
=3•2n-1 2n+1
,3 4
∴{dn}是等差数列.