The document discusses several angle theorems related to cyclic quadrilaterals: 1) Opposite angles of a cyclic quadrilateral are supplementary. This is proven using properties of angles on a circumference. 2) The exterior angle of a cyclic quadrilateral is equal to the opposite interior angle. 3) Angles subtended at the circumference by the same or equal arcs are equal. This is proven using properties of angles on a circumference.