An angle is the union of two non- collinear rays with a common initial point.


Two rays forming an angle are called the 'arms' of the angle and the common initial point is called the 'vertex' of the angle. sometimes, it will be convenient to refer to angle BAC, simply angle A. However this can not be done if there are more than one angle, with the same vertex A.



Interior of an angle: The interior of an angle BAC, is the set of all points P in its plane, which lie on the same side of line AB as C, and also on the same side of line AC as B.

Exterior of an angle: The exterior of an angle BAC is the set of all points Q in its plane, which do not lie on the angle or in its interior.

Types of Angles
An angle whose measure is 90 degrees is called a right angle.
An angle whose measure is less than 90 degree is called an acute angle.
An angle whose measure is more than 90 degrees is called an obtuse angle.