Circle Theorems

• The angle in a semi circle is 90°
• The angle at the circumference is half the angle at the centre
• The angles in the same segment from a common chord are equal
• Opposite angle in a cyclic quadrilateral always add to 180°
• The angle between a radius and a tangent is 90°
• Alternate segment theorem

  the angle between the chord and the tangent is equal to opposite angle in the triangle
• Tangents to a circle from the same point will be equal length
• The radius through the midpoint of a chord will bisect the chord at 90°