Show that $ \begin{vmatrix} 1 & 1 & 1 \\ \sin\alpha & \sin\beta & \sin\gamma \\ \cos\alpha & \cos\beta & \cos\gamma \end{vmatrix} = $`-4sin\frac{\alpha-\beta}{2}sin\frac{\beta-\gamma}{2}sin\frac{\gamma-\alpha}{2}`