- Circle TheoremsCircle Theorem 1: Angle at the Centre and CircumferenceCircle Theorem 2: Angle in a Semi-CircleCircle Theorem 3: Angles on the Same ArcCircle Theorem 4: Tangent and RadiusCircle Theorem 5: Isosceles TrianglesCircle Theorem 6: Interior Angles Cyclic QuadrilateralsCircle Theorem 7: Exterior Angles Cyclic Quadrilaterals