已知数列{an}的前n项和为Sn,且满足a1=1/2,an+2SnSn-1=0(n≥2). (Ⅰ)问:数列{1/Sn}是否为等差数列?并证明你的结论; (Ⅱ)求Sn和an.
问题描述:
已知数列{an}的前n项和为Sn,且满足a1=
,an+2SnSn-1=0(n≥2).1 2
(Ⅰ)问:数列{
}是否为等差数列?并证明你的结论;1 Sn
(Ⅱ)求Sn和an.
答
}是以2为首项,2为公差的等差数列.证明如下:
∵n≥2时,an+2SnSn-1=0,∴Sn-Sn-1+2SnSn-1=0
∴
-
=2
∵a1=
,∴
=2
∴数列{
}是以2为首项,2为公差的等差数列;
(Ⅱ)由(Ⅰ)知
=2+2(n-1)=2n,∴Sn=
;
∵n≥2时,an+2SnSn-1=0,
∴an=-2×
×
=
∴an=
.
(Ⅰ)数列{
1 |
Sn |
∵n≥2时,an+2SnSn-1=0,∴Sn-Sn-1+2SnSn-1=0
∴
1 |
Sn |
1 |
Sn−1 |
∵a1=
1 |
2 |
1 |
S1 |
∴数列{
1 |
Sn |
(Ⅱ)由(Ⅰ)知
1 |
Sn |
1 |
2n |
∵n≥2时,an+2SnSn-1=0,
∴an=-2×
1 |
2n |
1 |
2(n−1) |
1 |
2n(1−n) |
∴an=
|