show that the following function is continuos at (0,0)
$\displaystyle
f(x,y) = \left\{ \begin{array}{l}
\frac{{xsen(x)}}{{\sqrt {x^2 + y^2 } }}\,\,\,if(x,y) \ne (0,0) \\
0\,\,\,if\,(x,y) = (0,0) \\
\end{array} \right.
$
i need to know how to do this kind of problems, with this example i think i may be able to do other problems of the same kind
thanks