SI unit of plane angles, an alternative unit to the
degree. It is the angle at the centre of a circle when the centre is joined to the two ends of an arc (part of the circumference) equal in length to the radius of the circle. There are 2π (approximately 6.284) radians in a full circle (360°).
One radian is approximately 57°, and 1° is π/180 or approximately 0.0175 radians. Radians are commonly used to specify angles in
polar coordinates.
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