The sum of opposite angles in a cyclic quadrilateral

$$\begin{gathered} \mathrm{A} \mathrm{quadrilateral} \mathrm{having} \mathrm{all} \mathrm{its} \mathrm{vertices} \mathrm{on} \mathrm{a} \mathrm{circle} \mathrm{is} \mathrm{called} \mathrm{a} \mathrm{cyclic} \mathrm{quadrilateral} \\ \therefore \mathrm{ABCD} \mathrm{is} \mathrm{cuclic} \mathrm{quadrilateral} \\ \mathrm{The} \mathrm{sum} \mathrm{of} \mathrm{the} \mathrm{opposite} \mathrm{angles} \mathrm{of} \mathrm{a} \mathrm{cyclic} \mathrm{quadrilateral} \mathrm{is} {180}^0 \\ \mathrm{i}.\mathrm{e} \angle \mathrm{A}+\angle \mathrm{C}={180}^0 \mathrm{and} \angle \mathrm{B}+\angle \mathrm{D}={180}^0 \end{gathered}$$