公式： d(xy)=xdy+ydx d(lnx)=(1/x)dx d[ln(x-y)]=[1/(x-y)]d(x-y) 右侧： d[(x-y)(ln(x-y)] =d(x-y)ln(x-y)+(x-y)d[ln(x-y)] =(dx-dy)ln(x-y)+(x-y)/(x-y)d(x-y) =(dx-dy)ln(x-y)+d(x-y) =(dx-dy)ln(x-y)+(dx-dy)
