lim下面是n→∞∑上面是n下面是k=0(1/2)^k=lim(1+1/2+1/2^2+.+1/2^n)=lim(1-1/2^(n+1))/(1-1/2)=1/(1-1/2)=2(因为lim1/2^(n+1)=0)
