(1)设 $z= \ln \sqrt {x^{2}+y^{2}}$, 则 $x \dfrac { \partial z}{ \partial x}+y \dfrac { \partial z}{ \partial y}=\underline { \quad \quad \quad }$.

(2)设 $z= \arctan \dfrac {x+y}{1-xy}$, 则$ \dfrac { \partial z}{ \partial x}+ \dfrac { \partial z}{ \partial y}=\underline { \quad \quad \quad }$.