Given $$f(x,y)=\left\{ \begin{matrix} 0 & (x,y)=(0,0)\\ \frac {\sin\left(x^2-xy \right)}{\vert x \vert} & (x,y) \neq (0,0) \end{matrix} \right.$$ 

Proved continuity, I have to seek for differentiability. Calculating partial derivative in y by definition gives me a $0/0$ indetermination. Means it is not differentiable or are my calculus wrong?  

Thank you