an=Sn-S(n-1) (n>=2)=[1-(2/3)an]-[1-(2/3)a(n-1)]=-(2/3)an+(2/3)a(n-1)然后合并同类项,得(5/3)an=(2/3)a(n-1)an/a(n-1)=2/5a1=S1=1-(2/3)a1所以(5/3)a1=1,a1=3/5那么{an}就是以3/5为首项,2/5为公比的等比数列,则Sn...