已知数列{an}的前n项和为Sn,且Sn=2an-2(n∈N*).(1)求数列{an}的通项公式;(2)令bn=n?an+log 12an

2025-05-14 11:57:17
推荐回答(1个)
回答1:

(1)当n=1时,a1=2a1-2,解得a1=2;
当n≥2时,an=Sn-Sn-1=2an-2-(2an-1-2)=2an-2an-1
∴an=2an-1
故数列{an}是以a1=2为首项,2为公比的等比数列,
an=2?2n?1=2n
(2)由(1)得,bn=n?2n+log

1
2
2n=n?2n-n,
∴Tn=b1+b2+…+bn=(2+2?22+3?23+…+n?2n)-(1+2+…+n),
Rn=2+2?22+3?23+…+n?2n
2Rn22+2?23+3?24+…+n?2n+1
两式相减得-Rn=2+22+23+…+2n-n?2n+1=
2(1?2n)
1?2
?n?2n+1

Rn=(n?1)2n+1+2,
故Tn=b1+b2+…+bn=(n-1)2n+1+2-
n(n+1)
2

又由(1)得,Sn=2an-2=2n+1-2,
不等式(n-1)(Sn+2)-Tn<t+
19
32
n2 即为(n-1)2n+1-(n-1)2n+1-2+
n(n+1)
2
<t+
19
32
n2
,即为t>-
3
32
n2
+
1
2
n?2
对任意n∈N*恒成立,
设f(n)=-
3
32
n2
+
1
2
n?2
,则f(n)=-
3
32
(n?
8
3
)2?
4
3

∵n∈N*,∴f(n)max=f(3)=-
43
32

故实数t的取值范围是(-
43
32
,+∞
).