How does the arc length equal the radian measure of angle AOB ? (question 2 on page 271)

# For question 5 on page 242(Angles, Triangle and Polygons)…

For question 5 on page 242(Angles, Triangle and Polygons), could you explain why the triangle was put into 60,30,90 right triangle and how you came up with b/2 *square root of 3 as the height?