\begin{equation*}
\begin{split}
& \sin (-225^{\text{o}}) \cos (-180)^{\text{o}} \\\\
& -\sin 225^{\text{o}} \cos 180^{\text{o}} \\\\
& -\sin (180 + 45)^{\text{o}} \cos (180 - 0)^{\text{o}} \\\\
& - (- \sin 45^{\text{o}}) \:.\: - \cos 0^{\text{o}} \\\\
& \frac 12 \sqrt{2} \:.\: -1 \\\\
& -\frac 12 \sqrt{2}
\end{split}
\end{equation*}
