Additional Information A semicircle (or semicircle) is simply onehalf of circle. It can also be thought of as a sector with an angle of 180 degrees. If the perimeter of the semicircle is needed, add together the lengths of onehalf of the circumference + diameter 
 Where (for brevity) it says 'radius', 'circumference' and so on, it should, more correctly, be something like 'length of radius' or 'size of circumference' etc.
