The area of a semicircle is (?*R*R)/2. This means the radius of the semicircle is squared, multiplied by the constant pi, then divided by 2.
A semicircle is half a circle. The diameter of the circle divides the circle into two equal semicircles. The formula for a semicircle is derived from the formula of the circle, only divided by two. An alternative method is to use the diameter, which is twice the length of the radius. Therefore, using the diameter of the circle, the formula for finding the area of one semicircle is (?*D*D)/4, where D is the diameter and is equal to 2R.