Circle Theorem 1
Angle at centre is twice angle at circumference.
Circle Theorem 2
Angle at circumference of a semicircle is a right angle (90�).
Circle Theorem 3
Angles in same segment are equal.
Circle Theorem 4
Opposite angles in a cyclic quadrilateral is 180�.
Circle Theorem 5
Tangent to a circle is perpendicular to the radius.
Circle Theorem 6
Tangents to a circle from an external point to points of contact are equal length.
Circle Theorem 7
Line joining external point to centre of circle bisects angle between tangents.
Circle Theorem 8
Radius bisects chord at 90�.
Circle Theorem 9
Alternate segment theorem.