已知数列{an}的前n项和为Sn,a1=1,an+1=Sn+3n+1(n∈N*).(1)求数列{an}的通项公式;(2)令bn=2nana

2025-01-28 20:18:56
推荐回答(1个)
回答(1):

(1)∵a1=1,an+1=Sn+3n+1(n∈N*),①
∴当n≥2时,an=Sn-1+3(n-1)+1②
①-②得an+1-an=an+3,即n≥2时,an+1=2an+3,
又a2=S1+4=5=2a1+3,故对一切正整数n,an+1=2an+3,
则有an+1+3=2(an+3),所以数列{an+3}是公比为2,首项为a1+3=4的等比数列,
故an+3=4?2n-1
∴an=2n+1-3(n∈N*).
(2)bn=

2n
anan_+
1=
1
2
?
2n^+
^
1
=
1
2
?
(2n^+
^
2
=
1
2
1
2n^+
1
?3
-
1
2n^+
2
?3
),
故Tn=b1+b2+…+bn=
1
2
[(
1
22?3
-
1
23?3
)+(
1
23?3
-
1
24?3
)+…+(
1
2n^+
1
?3
-
1
2n^+
2
?3
)]
=
1
2
×(
1
22?3
-
1
2n^+
2
?3
)=