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