#### Answer

{$(x,y) | (x,y) \neq (0,0)$}

#### Work Step by Step

From the given function $f(x,y)$ we can see that it represents a represents a rational function which is continuous on its domain $D$ except at $(0,0)$.
Thus, the limit does not exist along the line $y=0$, when $(x,y)$ approaches to $0$ which cannot be the value of the function. Therefore, we can conclude that $f(x,y)$ is not continuous at $(0,0)$.
This mean that $(x,y) \neq (0,0)$
Hence, {$(x,y) | (x,y) \neq (0,0)$}