To watch a video animation go to: https://youtu.be/dg7s1ODk9sY

1. Circles
Circle theorems
The angle at the centre
is twice the angle at
the circumference.
The angle between a tangent
and a chord is equal to the
angle in the alternate segment.
Opposite angles in a
cyclic quadrilateral
add up to 180°.
Tangents from a point
are equal in length.
The angle between a tangent
and the radius is 90°.
The perpendicular line from
the centre of a circle to a
chord bisects the chord.
Angles in the same segment
are equal.
The angle within a
semicircle is 90°.