A radius forms a right angle with a chord at the midpoint of the chord; Two tangents drawn to a circle from the same point outside the circle are equal in length; A tangent is at right angles to the radius through the point of contact.

Every angle in a semi-circle is a right angle.

The angle at the center is twice the angle at the circumference.

Angles in the same segment of a circle are congruent.

The angle between a tangent to a circle and a chord drawn from the point of tangency is equal to an angle in the alternate segment.