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Two rays with a common starting point from an angle. One of the rays of angle is called initial side and the other as terminal side. The angle is identified by showing the direction of rotation from the initial side to the terminal side.
An angle is said to be positive/negative if the rotation is anti-clockwise/ clockwise. Angles are usually denoted by Greek letters such as  (alpha),  (beta),  (gamma),  (theta) etc.
In the figure below  is a positive and  is negative.
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