The radian is the SI supplementary unit of angle. It is defined as the angle subtended at the center of a circle by an arc of circumference equal in length to the radius of the circle.
There are 2π (about 6.283185) radians in a complete circle. One radian is equal to 180^{o}/π or approximately 57.29578^{o}.
In calculus, the use of radians leads to the simple identity
For measuring solid angles, see steradian.
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