设公比为q则a2=a1*q,a3=a1*q^2
又an满足a1a2=1/3,a3=1/9
所以a1^2*q=1/3,a1*q^2=1/9
两式相乘得,a1^3*q^3=1/27,a1*q=1/3=a2
所以a1=1
故通项公式an=1/3^(n-1)