radian
A unit for measuring angles where the radius is equal to the length of the intercepted arc
A radian is a unit of measurement for angles. It is defined as the angle subtended at the center of a circle by an arc with a length equal to the radius of the circle. In other words, a radian is the measure of an angle that, when its vertex lies at the center of a circle, the length of the arc cut by the angle on the circumference of the circle is equal to the radius of the circle. The symbol for radian is rad. One radian is equal to approximately 57.3 degrees.
Radian measurement is commonly used in trigonometry, calculus, and physics as it simplifies many mathematical calculations. It is the preferred unit of measurement when dealing with angles involving calculus, as the trigonometric functions have simpler and more elegant definitions when using radian measure.
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