1/3+1/6+1/12+1/24+...+1/[3×2^(n-1)]=1/3× [ 1+1/2+1/4+1/8+...+1/2^(n-1) ]=1/3×[1-(1/2^n)]/(1-1/2)=1/3×[1-(1/2^n)]×2=2/3×[1-(1/2^n)]
