已知数列{an}的首项a1=1,前n项之和Sn满足关系式:3tSn+1-(2t+3)Sn=3t(t>0,n∈N*).(1)求证:数

2025-05-04 18:42:14
推荐回答(1个)
回答1:

(1)证明:∵3tSn+1-(2t+3)Sn=3t,
∴3tSn-(2t+3)Sn-1=3t,(n≥2),
两式相减得3tan+1-(2t+3)an=0,
又∵t>0,∴

an+1
an
2t+3
3t
(n≥2),
当n=2时,3tS2-(2t+3)S1=3t,
即3t(a1+a2)-(2t+3)a1=3t,且a1=1,
得a2=
2t+3
3t
,则
a2
a1
2t+3
3t

an+1
an
2t+3
3t
对n≥1都成立,
∴{an}为以1为首项,
2t+3
3t
为公比的等比数列,
(2)解:由已知得,f(t)=
2t+3
3t

bn+1=f(
1
bn
)
=
2
bn
+3
3
bn
=
2+3bn
3
=
2
3
+bn

bn+1?bn
2
3

∴{bn}是一个首项为1,公差为
2
3
的等差数列,
则bn=1+(n-1)×
2
3
=
2
3
n+
1
3

(3)解:Tn=b1b2-b2b3+b3b4-…+b2n-1b2n-b2nb2n+1?
=b2(b1-b3)+b4(b3-b5)+…+b2n(b2n-1-b2n+1)=-2d(b2+b4+…+b2n
=-2×
2
3
(b2+b4+…+b2n)=-2×
2
3
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