Circle theorems

Circle theorems

1. Basic theorems

   • The radius is the line that goes from the centre to the circumference.
   • The diameter is the line across the middle of the circle.
   • The circumference is the line on the outside of the circle

2. Pi ∏

   • to find the circumference you use ∏d
     • ∏r2 = Pi time radius squared
   • to find the area you use ∏r2
     • ∏d = Pi times the diameter

3. Complecated theorems

   • Subtended angles
     • The angle subtended at the circumference is half the angle subtended at the centre
   • opposite angles in a quadrilateral
     • opposite angles in a quadrilateral a equal
   • cyclic quadrilateral
     • it is only a cyclic quadrilateral if a the points touch the circumference
   • tangents
     • this is when a STRAIGHT LINE touches the circumference
     • it always forms a 90° angle

4. isosolie triangles

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