2an=S(n)S(n-1) 2(Sn-Sn-1)=S(n)S(n-1) 2/Sn-1-2/Sn=1 1/Sn-1/Sn-1=-1/2 {1/Sn}是公差为-1/2的等差数列 1/Sn=1/S1-(n-1)/2 Sn=6/(5-3n) Sn-1=6/(8-3n)
