f(x,y)= \left\{ \begin{array}{rcl}
 
(x+y)sin\frac{1}{x}sin\frac{1}{y} & \mbox{za} & (x,y) \in D_f \right \\
 
0 & \mbox{za} & (x,y)=(0,0);(x,y)=(0,\frac{1}{k\pi});(x,y)=(\frac{1}{k\pi},0). k\in Z \right \\ 
 
\end{array}\right.