A tangent (a line touching the circumference) meets the radius at 90 degrees

The angle at the circumference is half the size of the angle at the center

Angles in the same segment are equal

The angle drawn from two ends of a diameter and touching the circumference is 90 degrees

Opposite angles in a cyclic quadrilateral (a shape with all 4 sides touching the circumference)add to 180 degrees

Tangents from a point outside the circle are equal in length (icecream theorem :3 )

The angle between the tangent and chord at the point of contact is equal to the angle in the alternate segment (Alternate segment theorem)