If $u=\large\frac{1}{\sqrt{x^{2}+y^{2}}},$ then $x\large\frac{\partial u}{\partial x}$$+y\large\frac{\partial u}{\partial y}$ is equal to

Question

$$\begin{array}{1 1}(1)\frac{1}{2} u&(2)u\\(3)\frac{3}{2} u&(4)-u\end{array}$$