## Trigonometry (10th Edition)

If a radius with a length of $r$ units rotates through an angle of 1 radian, then the arc length is $r$ units. An angle of 1 radian subtends an arc length that is equal to the radius.
Radian measure is a unit that is used to measure an angle. If a radius with a length of $r$ units rotates through an angle of 1 radian, then the arc length is $r$ units. An angle of 1 radian subtends an arc length that is equal to the radius. We can use the conversion factor $\frac{180^{\circ}}{\pi}$ to convert from radians to degrees. $(1~radian) \times \frac{180^{\circ}}{\pi} = 57.3^{\circ}$ An angle of 1 radian is equal to $57.3^{\circ}$