设S=1+1/2+1/4+...+1/2^(n-1) 2S=2+1+1/2+...+1/2^(N-2) S=2S-S =2+(1-1)+(1/2-1/2)+...+[1/2^(N-2)-1/2^(N-2)]-1/2^(n-1) =2-1/2^(n-1)
