<span>πd is the circumference of a circle, so with a diameter of 4 a single circle has a circumference of 4π. 1/3 of each circle is in contact with the belt, so that total part is still 4π. Then, the part of the belt connecting from tangent to tangent is 2 radii, so 3 areas is 12. The belt would then be 12+4π cm.</span>
