12. Prove that a cyclic parallelogram is a rectangle.
Thus, $$\angle{P}$$ + $$\angle{R}$$ = $$180^\circ$$ ...(i)(Since, Sum of opposite angles in a cyclic quadrilateral is $$180^\circ$$)
But, $$\angle{P}$$ = $$\angle{R}$$ ...(ii)(Since, in a parallelogram, opposite angles are equal)
$$\angle{P}$$ = $$\angle{R}$$ = $$90^\circ$$
Similarly, $$\angle{Q}$$ = $$\angle{S}$$ = $$90^\circ$$
Thus, Each angle of PQRS is $$90^\circ$$.