could you help me on the following,
rcosxcosy * rsinxcosy
-rsinxsiny * sinxsiny
try using the following identities:
$\displaystyle
\begin{gathered}
\cos ^2 x - \sin ^2 x = \cos 2x \hfill \\
2\sin x\cos x = \sin 2x \hfill \\
\sin ^2 x + \cos ^2 x = 1 \hfill \\
\end{gathered}
$
you might also take a look at this:
Table of Trigonometric Identities