## Precalculus (6th Edition) Blitzer

The radian measure of $\theta$ shown in the figure is $\theta =$ $\frac{s}{r}$.
If $s$ is the length of an arc in a circle and $r$ is the radius of the circle, then the angle $\theta$ that intercepts the arc is given by $\theta =\frac{s}{r}\ \text{radians}$ Thus, the radian measure of $\theta$ shown in the figure is $\theta =$ $\frac{s}{r}$.