若数列{an}的前n项和Sn满足2Sn=3an-1(n∈N*),等差数列{bn}满足b1=3a1,b3=S2+3.(1)求数列{an}、{bn

2025-02-24 01:01:01
推荐回答(1个)
回答1:

(1)当n=1时,2S1=3a1-1,∴a1=1,
当n≥2时,2an=Sn-2Sn-1=(3an-1)-(3an-1-1),即an=3an-1
∵a1=1≠0,
∴数列{an}是以a1=1为首项,3为公比的等比数列,∴an3n?1
设{bn}的公差为d,b1=3a1=3,b3=S2+3=7=2d+3,d=2.
∴bn=3+(n-1)×2=2n+1;
(2)∵cn=
bn
3an
=
2n+1
3n

Tn
3
31
+
5
32
+
7
33
+…+
2n+1
3n
  ①
1
3
Tn
3
32
+
5
33
+
7
34
+…+
2n+1
3n+1
  ②
由①-②得,
2
3
Tn=1+
2
32
+
2
33
+
2
34
+…+
2
3n
?
2n+1
3n+1

=1+2×
1
9
(1?(
1
3
)n?1)
1?
1
3
?
2n+1
3n+1

Tn=2?
n+2
3n