a(n+1)=2+2/3Snan=2+2/3S(n-1)相减得a(n+1)-an=2/3(Sn-S(n-1)=2/3an3a(n+1)-3an=2an3a(n+1)=5ana(n+1)/an=5/3所以是等比数列,公式是5/3则an=a1(q^n-1)/(q-1)=3((5/3)^n-1)/(5/3-1)=3(5^n*3^(-n)-1)/2/3=9(5^n*3^(-n)...