Use a trigonometric substitution:
<br />
\begin{gathered}<br />
\int {\sqrt {4 - x^2 } dx \Rightarrow } \left[ {\begin{array}{*{20}c}<br />
{x = 2\sin \theta } \\<br />
{dx = 2\cos \theta d\theta } \\<br />
<br />
\end{array} } \right] \hfill \\<br />
\int {2\cos \theta \sqrt {4(1 - \cos ^2 \theta )} } d\theta = 2\int 2 \sin \theta \cos \theta d\theta \cdot \cdot \cdot \hfill \\ <br />
\end{gathered} <br />
I think you can get it from there.