In mathematics and physics, radian is the unit of measurement of angle. It is a unit derived from the international system of units, abbreviated as rad. Definition: The central angle of an arc whose arc length is equal to the radius of a circle is 1 radian. According to the definition, the number of radians in a week is 2π r/r = 2π, and the angle of 360 = 2π radians, so 1 radian is about 57.3. 1 is π/ 180 radian, approximate value is 0.0 1745 radian, fillet is 2π radian, straight angle (i.e.180 angle) is π radian, and right angle is π/2 radian. In concrete calculation, when the angle is given in radians, radian units are usually not written directly. We have learned the formula of arc length: l=nπr/ 180, where n is the number of angles. But if radians are used, the above formula becomes simple: (note that radians are positive and negative) l=|α| r, which is the product of the size and radius of α. Similarly, we can simplify the formula of sector area in the scientific calculation method of calculator program attached to Windows operating system (starting from the lower left corner of the computer → program → attachment → calculator), and we can call radian to calculate. The number of special angles and radians corresponds to the scale of 0 30 45 60 90120135150 65448.

The radian is 0 π/6 π/4 π/3 π/22 π/33 π/45 π/6 π 3 π/22 π.