（2+ln(x-y)）/（3+ln(x-y)）=x/(2x-y)(2x-y)[2+ln(x-y)]=x[3+ln(x-y)]4x-2y+(2x-y)ln(x-y)=3x+xln(x-y)(x-y)ln(x-y)=2y-x
ln(x-y)带进去就行
