已知正项数列{an}的前n项和为Sn,且an和Sn满足4Sn=(an+1)2(n=1,2,3…).(Ⅰ)求{an}的通项公式;

2025-05-14 21:51:13
推荐回答(1个)
回答1:

(I)当n=1时,4S1=4a1=(a1+1)2,解得a1=1.
当n≥2时,4an=4(Sn-Sn-1)=(an+1)2?(an?1+1)2,化为(an+an-1)(an-an-1-2)=0,
∵正项数列{an},∴an-an-1=2.
∴数列{an}为等差数列,an=1+2(n-1)=2n-1.
(II)bn=

1
anan+1
=
1
(2n?1)(2n+1)
=
1
2
(
1
2n?1
?
1
2n+1
)

∴{bn}的前n项和Tn=
1
2
(1?
1
2n+1
)
=
n
2n+1

(III)对任意n∈N*,Tn
m
32
都成立,
∴m<32×
1
2
(1?
1
2n+1
)

∵数列{1?
1
2n+1
}
是单调递增数列,因此当n=1时,取得最小值
32
3

m<
32
3

∴整数m的最大值为10.