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
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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
=an+1成立,cn nbn
∴当n=1时,
=a2,c1 b1
∵a1=3,b1=1,
∴c1=3,S1=3;
当n≥2时,∵
+c1 b1
+…+c2 2b2
=an+1,cn nbn
∴
+c1 b1
+…+c2 2b2
=an,cn-1 (n-1)bn-1
∴
=an+1-an=2,cn nbn
∴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-
)•3n+1 2
,3 2
又S1=3满足上式,
综上,Sn=(n-
)•3n+1 2
,n∈N*.3 2