Let Where Is $f$ Continuous

Short Answer

Let $f ( x , y ) = \left\{ \begin{array} { l l } \frac { x ^ { 4 } - y ^ { 4 } } { x ^ { 2 } + y ^ { 2 } } & \text { if } ( x , y ) \neq ( 0,0 ) \\0 & \text { if } ( x , y ) = ( 0,0 )\end{array} \right.$ Where is $f$ continuous?