Angles - Basic Terms and Definitions


Line-segment: A line with two endpoints.
Ray: Part of a line with one endpoint called a ray.
Collinear points: If three of more points lie on the same line, they are called collinear points.
Non-collinear point: If three of more points not lie on the same line.
Angle: An angle is formed when two rays are originated from the same end point.
The rays making an angle are called the arms of the angle and the end point is called the vertex of the angle.

Acute angle: Angles greater then zero degree and less then 90 degree.
Right angle: Angles equal to 90 degree.
Obtuse angle: Angles greater then 90 degree and less then 180 degree.
Straight angle: Angle equal to 180 degree.
Reflex angle: Angle greater then 180 degree and less then 360 degree.

Complementary angles: Two angles whose sum is 90 are called complementary angles.
Supplementary angle: Two angles whose sum is 180 are called supplementary angles.

Adjacent Angles: Two angles are adjacent, if they have a common vertex, a common arm and their non-common arms are on different sides of the common arm.
Angle ABD and angle DBC are adjacent angles. Ray BD is their common arm and point B is their common vertex. Ray BA and ray BC are non common arms. Moreover, when two angles are adjacent, then their sum is always equal to the angle formed by the two noncommon arms.

linear pair of angles: If non common arm in adjacent angles form a line then angles are called linear pair of angles.
Vertically opposite angles: Vertically opposite angles are formed when two lines intersect each other.


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