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radian(ray -dee-ăn) A unit of angle used in plane geometry that is the angle subtended at the center of a circle by an arc of the circle equal in length to the circle's radius. Thus 2π radians equals 360°, 1 radian equals 57°.296, and 1° equals 0.017 radians. Solid angles, used in spherical geometry, are measured in steradians.
the central angle subtended in a circle by an arc whose length is equal to the circle’s radius. A radian is approximately equal to 57°17′44.8″. The radian is used as the unit of measurement of angles in what is known as circular, or radian, measure. If the circular measure of an angle is a radians, the angle contains (180a/π)°. Conversely, if the degree measure of an angle is n°, the circular measure is πn/180 radians. For example, angles of 30°, 45°, 60°, 90°, and 180° correspond to angles of π/6, π/4, π/3, π/2, and π radians, respectively.