首先∂z /∂x=f1' *∂(x+y)/x +f2' *∂x/∂x=f1' +f2'那么再继续求二阶偏导数，得到∂^2 z/∂x^2=∂(f1' +f2') /∂x=f11" +f12" +f21" +f22"=f11" + 2f12" +f22"
