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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 180o/π or approximately 57.29578o.

In calculus, the use of radians leads to the simple identity

<math>\lim_{h\rightarrow 0}\frac{\sin h}{h}=1</math>
which is the basis of many elegant identities in mathematics.

For measuring solid angles, see steradian.

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