The radian, in mathematics, is a unit of plane angle, equal to 180/? degrees, or about 57.2958 degrees. It is represented by the symbol "rad" or, more rarely, by the superscript c (for "circular measure"). For example, an angle of 1.2 radians would be written as "1.2 rad" or "1.2^c" (the second symbol can be mistaken for a degree: "1.2°"). However, the radian is the standard unit of angular measurement for mathematicians, and in mathematical writing the symbol "rad" is almost always omitted. In the absence of any symbol radians are assumed, and when degrees are meant the symbol ° is used. The radian was formerly an SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an SI derived unit. The SI unit of solid angle measurement is the steradian. Definition of the circle.]] One radian is the angle subtended at the center of a circle by an arc of circu

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