## Quick Reference

A method of measuring angles by treating them as the angle formed by a sector of a circle at the circle's centre. The unit of measure is the **radian**, the angle subtended at the centre of a circle by an arc of equal length to the radius. Since an arc of length *r* subtends an angle of 1 radian, the whole circumference, length 2π*r*, will subtend an angle of 2π*r*/*r* = 2π radians. Thus, 360° = 2π radians; 1 radian = 57.296°.

*Subjects:*
Physics.