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设数列{an}的前n项和为Sn=2n2,{bn}为等比数列,且a1=b1,b2(a2-a1)=b1
(Ⅰ)求数列{an}和{bn}的通项公式;
(Ⅱ)设cn=
an
bn
,求数列{cn}的前n项和Tn
人气:327 ℃ 时间:2019-08-18 18:16:07
解答
(1):当n=1时,a1=S1=2;当n≥2时,an=Sn-Sn-1=2n2-2(n-1)2=4n-2,
故{an}的通项公式为an=4n-2,即{an}是a1=2,公差d=4的等差数列.
设{bn}的公比为q,则b1qd=b1,d=4,∴q=
1
4

故bn=b1qn-1=2×
1
4n-1
,即{bn}的通项公式为bn=
2
4n-1

(II)∵cn=
an
bn
=
4n-2
2
4n-1
=(2n-1)4n-1
Tn=c1+c2+…+cn
Tn=1+3×41+5×42+…+(2n-1)4n-1
4Tn=1×4+3×42+5×43+…+(2n-3)4n-1+(2n-1)4n
两式相减得,3Tn=-1-2(41+42+43+…+4n-1)+(2n-1)4n=
1
3
[(6n-5)4n+5]
∴Tn=
1
9
[(6n-5)4n+5]
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