$$\left. \begin{array} { l } { \cos ( A - D ) \sin ( B - C ) + \cos ( B - D ) \sin ( C - A ) + \cos ( C - D ) \sin ( A - B ) = 0 } \end{array} \right.$$
$A\in \mathrm{R}$
$D\in \mathrm{R}$
$B\in \mathrm{R}$
$C\in \mathrm{R}$
