Question

已知数列{an}的前n项和为Sn,且满足a1=1/2,an+2SnSn-1=0(n>=2)

Answer 1

1.sn-s(n-1)+2SnSn-1=0 1/sn -1/s(n-1)=2 所以1/sn是以1/s1=2为首项 公差为2的等差数列 即

sn=1/2n an=-1/n(2n-2)(n≥2) bn=1/n bn^2=1/n^2