In geometry, a pair of rays (see line) sharing a common endpoint (the vertex). An angle may be thought of as the rotation of a single ray from an initial to a terminal position. Clockwise rotation is considered negative and counterclockwise rotation positive. Either may be measured in degrees (one full rotation = 360°) or radians (one full rotation = 2 rad). A 90° angle is called a right angle. Any angle less than 90° is an acute angle. Any angle more than 90° but less than 180° is an obtuse angle.

This entry comes from Encyclopædia Britannica Concise.
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