Radian
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A radian is the angle subtended by an arc length equal to the radius of a circle. There are 2π radians in a full circle, as the arc length of the circumference of a circle is equal to 2π times the radius of the circle.
Measuring angles in radians often allows the direct use of angles in mathematical formulae, particularly in trigonometric functions and their expansions.
The radian is the supplementary unit of angular measure in the International System of Units.