Circle Theorems

• Angles in the same segment are equal

  
• Angles in a cyclic quadrilateral add to 180

  
• The angle at the centre is double the angle at the circumference when subtended by the same arc / chord

  
• The angle in a semicircle is 90

  
• The perpendicular bisector of a chord passes through the centre OR A diameter / radius meets a chord at 90

  
• A radius meets a tangent at 90

  
• Two tangents from the same external point are equal in length

  
• Alternate segment theorem