Let $ABCD$ be a parallelogram. Let $O$ be a point in its interior such that $\angle AOB + \angle DOC = 180^{\circ}$. Show that $\angle ODC = \angle OBC$.
Let $ABCD$ be a parallelogram. Let $O$ be a point in its interior such that $\angle AOB + \angle DOC = 180^{\circ}$. Show that $\angle ODC = \angle OBC$.