Using Euler's theorem prove the following: if $u$ is a homogenous function of $x$ and $y$ of degree$\;n$, prove that$\;x\large\frac{\partial^{2} u}{\partial x^{2}}$+$y\large\frac{\partial^{2}u}{\partial x\partial y}$=$(n-1)\large\frac{\partial u}{\partial x}$