Circle Theorems

• Semi circles

  A semi circle always has a right angle (90°).
• Arrowhead

  Angle at origin is twice the angle at the circumference.
• Bow-tie

  Angles in the same segment are equal.
• Cyclical quadrilaterals

  Opposite angles in a cyclical quadrilateral sum to 180°.
• Tangent and radius

  A tangent is perpendicular to a radius.
• Tangents at single point

  Tangents from a single point are equal in length.
• Alternate segments

  Angles in alternate segments are equal.
• Chords

  A radius bisects a chord perpendicularly.