1/（x-y）-1/（y-x）=1/（x-y）-【-1/-（y-x）】=1/（x-y）+1/（x-y）=2/（x-y）
1/(x-y)-1/(y-x)=1/(x-y)-1/[-(x-y)]=1/(x-y)+1/(x-y)
