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The radian is a natural measure of angle.

The circumference of a circle of radius <math>r</math> is <math>2 \pi r</math>. The angle <math>\theta</math> at the centre of a circle, in radians, is defined to be equal to the length of arc subtended by that angle divided by the circle's radius.

By this definition, any complete circle subtends an angle of <math>2\pi</math> radians.

See also