Worlds of David Darling
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My geometry teacher was sometimes acute, and sometimes obtuse, but he was always right.

types of angles
Types of angle
In plane geometry, an angle is the opening between two lines or two planes that meet; the word comes from the Latin angulus for a sharp bend. Plane angles are measured in degrees or radians.

A right angle has exactly 90° (or π/2 radians), an acute angle less than 90°, an obtuse angle has between 90° and 180°, and a straight angle has exactly 180°. Any angle that is at most 180° is said to be convex. An angle between 180° and 360° is called a reflex angle. An angle of exactly 360° is said to be a full angle.

Complementary angles add to give 90°, supplementary angles make a total of 180°. Adjacent angles are next to, and thus share, one side.

A reentrant angle is an inward-pointing angle of a concave polygon.

In spherical geometry, a spherical angle is that formed by intersecting arcs of two great circles; its magnitude is equal to that of the angle between the two planes of the great circles.

See also solid angle.


A protractor is an instrument for measuring angles. Usually semicircular, it is marked off in degrees along the semicircular edge.


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