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Radian SI Supplementary Unit - The radian is the plane angle between two radii of a circle which cut off on the circumference an arc equal in length to the radius.
The circumference of a circle is defined as 2π*radius. Considering a unit circle centred on a Cartesian axis. By tracing the x-coordinate (starting at (x=1,y=0) and travelling counter-clockwise) versus arc length we obtain a Cosine wave and similary the y-coordinate yields a Sine wave. From this derives the definition that 360° equals 2π radians.
See also: Angles, Plane Angle, SI Units.
  
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