arc length s is given by s = r*theta, where theta is the central angle in radians (not degrees).
Here the diam. is 5 cm, so the radius is 5/2 cm. 60 deg = sqrt(3) / 2. Thus,
the arc length is s = (5/2 cm)*sqrt(3) / 2. This simplifies to (5/4) sqrt(3).
