倍数数列:a1=1/3 q=1/3
1/3+1/9+…+1/3^n=(a1-an*q)/(1-q)=(1/3-1/3^n+1)/(2/3)=((1-1/3^n)/2=1/2-1/3^n *2
lim(1/2-1/3^n *2)n→∞=1/2