a(n+1)=SnS(n+1)S(n+1)-Sn=SnS(n+1)等式两边同除以-SnS(n+1)1/S(n+1) -1/Sn=-1，为定值1/S1=1/a1=1/(-1)=-1数列{1/Sn}是以-1为首项，-1为公差的等差数列1/Sn=-1+(-1)(n-1)=-nSn=-1/n
20还详细:难怪数学不好
