$\displaystyle f(x,y) = \left\{ \begin{gathered}

\frac{{{x^2} - {y^2}}}

{{\sqrt {{x^2} + {y^2}} }}for(x,y) \ne (0,0) \hfill \\

1for(x,y) = 0 \hfill \\

\end{gathered} \right.

$

I tried multiplying the first part by sq rt x^2 + y^2 but I don't know where to continue from there