請鏈接下在網址：

熱心網友

直接求導z'(x)=[y(x+y)-xy]/(x+y)^2=y^2/(x+y)^2,z'(y)=x^2/(x+y)^2x*z'(x)+y*z'(y)=xy(x+y)/(x+y)^2=xy/(x+y)=z