问题 解答题
已知数列{an}是公差为d(d≠0)的等差数列,数列{bn}是公比为q的(q∈R)的等比数列,若函数f(x)=x2,且a1=f(d-1),a5=f(2d-1),b1=f(q-2),b3=f(q).
(1)求数列{an}和{bn}的通项公式;
(2)设数列{cn}的前n项和为Sn,对一切n∈N*,都有
c1
b1
+
c2
2b2
+…+
cn
nbn
=an+1
成立,求Sn
答案

(1)∵数列{an}是公差为d(d≠0)的等差数列,f(x)=x2,且a1=f(d-1),a5=f(2d-1),

∴(d-1)2+4d=(2d-1)2

∴d=2,a1=1.

∴an=2n-1;

∵数列{bn}是公比为q的(q∈R)的等比数列,f(x)=x2,且b1=f(q-2),b3=f(q),

则b2=q

∴q2=q2(q-2)2

解得q=3,或q=1,又b1=1.

∴bn=3n-1;或bn=1

(2)∵对一切n∈N*,都有

c1
b1
+
c2
2b2
+…+
cn
nbn
=an+1成立,

∴当n=1时,

c1
b1
=a2

∵a1=3,b1=1,

∴c1=3,S1=3;

当n≥2时,∵

c1
b1
+
c2
2b2
+…+
cn
nbn
=an+1

c1
b1
+
c2
2b2
+…+
cn-1
(n-1)bn-1
=an

cn
nbn
=an+1-an=2,

∴cn=2n•3n-1

故cn=

3,n=1
2n•3n-1,n≥2

∴Sn=c1+c2+…+cn

=3+2•2•3+2•3•32+2•n•3n-1

=2(1•30+2•31+3•32+n•3n-1)+1

设x=1•30+2•31+3•32+…+n•3n-1,①

则3•x=1•31+2•32+…+(n-1)•3n-1+n•3n,②

②-①得2x=n•3n-(3n-1+3n-2+…+30)=n•3n-

3n-1
2

∵sn=2x+1,

Sn=(n-

1
2
)•3n+
3
2

又S1=3满足上式,

综上,Sn=(n-

1
2
)•3n+
3
2
,n∈N*

单项选择题
单项选择题