The angle between the tangent and the chord is equal to the angle subtended by the chord in the opposite segment of the circle.
Alternate Segment Theorem
The angle subtended at the centre of a circle by an arc is twice the angle subtended at any point on the circumference by the same arc
Angle in the Same Segment Theorem
Angles subtended by the same chord in the same segment of a circle are equal
Angle in Semi Circle Theorem
An angle subtended by a diameter at the circumference of a circle is always 90°
Cyclic Quadilateral Theorem
In a cyclic quadrilateral (a four-sided shape where all vertices lie on the circumference of a circle), the sum of opposite angles is always 180°
