f(x,y)= \left\{ \begin{array}{rcl}
 
f(x,y)=\frac{1}{x}sin\frac{x^3}{x^2+y^2} & \mbox{za} & (x,y) \in D_f \right \\
 
0 & \mbox{za} & (x,y)=(0,y)\ y\neq 0 \right \\ 
 
\end{array}\right.