Question

An arc is 6.5 cm long and it subtends a central angle of 45 degrees. Find the radius of the circle.

Answer 1

Arc length = radius * central angle (measured in radians)
There are 57.2957795131 degrees per radian so
45 degrees = (45 / 57.2957795131) =  0.7853981634 radians
radius = arc length / central angle (radians)
radius = 6.5 / 0.7853981634 radians = 8.2760570408 cm

http://www.1728.org/radians.htm