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 An angle is formed by the intersection of two rays or line segments (called the sides) with a common endpoint (called the vertex).

 Naming Angles
 An angle is named using three letters, where the middle letter corresponds to the vertex of the angle. The angle at the right may be referred to as ∠ABC or ∠CBA. If it is perfectly clear which angle is being named, an angle may be referred to by its vertex letter alone, such as, in this case, ∠B. (See more about naming angles.) It is understood, unless otherwise stated, that angles are positive, counterclockwise, and less than or equal to 180º.

 Types of Angles
 An acute angle is an angle whose measure is less than 90º. A right angle is an angle whose measure is 90º.
An obtuse angle is an angle whose measure is greater than 90º but less than 180º.

A straight angle is an angle whose measure is 180º.

A reflex angle is an angle whose measure is greater than 180º but less than 360º
If an angle is a reflex angle, it will be clearly indicated by a drawing or by a reference in the wording of the problem.

An oblique angle is any angle that is not 90º, not a right angle.
 Oblique angle = NOT a Right Angle.
A dihedral angle is an angle between two planes.

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