We define \(f(x,y) = \frac{x^3-y^3}{x^2+y^2}\), unless \(x=y=0\), and \(f(0,0)=0\). If \(f_x(h,k)\) means the value of \(\frac{\partial f}{\partial x}\) at \(x=h, y=k\), find \(f_x(h, mh)\) for \(h \ne 0\), and \(f_x(0,0)\). For what values of \(m\) is \(\lim_{h \to 0} f_x(h,mh) = f_x(0,0)\)?