Theorem 1.20-1.23 Tests of Concyclic Points
Theorem 1.20 Any 3 non-collinear points are concyclic. Only one circle can be drawn passing through them.
Theorem 1.21 converse of angles in the same segment
Theorem 1.22 opp. angles supp.
Theorem 1.23 ext. angle = int. opp. angle