The Radian reference article from the English Wikipedia on 24-Apr-2004
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Radian

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The radian is the SI derived 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.

An angle measuring 1 radian subtends an arc equal in length to the radius of the circleEnlarge

An angle measuring 1 radian subtends an arc equal in length to the radius of the circle

There are 2&pi (about 6.283185) radians in a complete circle. One radian is equal to 180°/π or approximately 57.29578°.

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

which is the basis of many elegant identities in mathematics.

Conversion of degrees to radians can be carried out using the formula

The radian was formerly an SI supplementary unit, but this catagory was abolished from the SI system in 1995.

For measuring solid angles, see steradian.