If $z=x . f\left(\frac{y}{x}\right)+F\left(\frac{y}{x}\right)$, prove that:
(a) $x \frac{\partial z}{\partial x}+y \frac{\partial z}{\partial y}=z-F\left(\frac{\ell}{x}\right)$
(b) $x^{2} \frac{\partial^{2} z}{\partial x^{2}}+2 x y \frac{\partial^{2} z}{\partial x \cdot \partial y}+y^{2} \frac{\partial^{2} z}{\partial y^{2}}=0$