1-1/2+1/3-¼+...+1/(2n-1)-1/2n=1/(n+1)+1/(n+2)+...+1/2n
1-1/2+1/3-1/4+...+1/(2n-1)-1/2n==1+1/2+1/3+1/4+...+1/(2n-1)+1/2n-2(1/2+1/4+...+1/2n)==1+1/2+1/3+1/4+...+1/(2n-1)+1/2n-(1/1+1/2+...+1/n)==1/(n+1)+1/(n+2)+...+1/2n
