当n=>∞时收敛于 S=ln2 1-1/2+1/3-1/4……+1/2n =1+1/2+1/3+1/4……+1/2n-2(1/2+1/4+……+1/2n) =1/(n+1)+1/(n+2)+……1/2n =1/n(1/(1+1/n)+1/(1+2/n)+……+1/(1+n/n) =1/(1+x)[从0积到1] =ln2