The Angle x subtended at the centre of a circle by an arc is twice the size of the angle on the circumference subtended by the same arc.
Angle subtended at the Centre is twice the angle at the circumference
Angles Subtended in the same segment
of a circle are equal
Points which lie on the circumference of the
same circle are called cyclic (or concyclic)
points. A cyclic quadrilateral is a quadrilateral
with all its four corners (vertices) on the
circumference of the same circle.
OBT and OAT
The Shaded Segment BED is called the alternate segment to the angle CBD
The angle between a tangent to a circle and a chord drawn through the point
of contact is equal to any angle subtended by the chord at the circumference in
the alternate segment
The angle in a semi circle is 90 degrees!