Area of a circle is pi x r^2
R is the radius which is have the diameter: r = 10/2 = 5
Area of the full circle would be 3.14 x 5^2 = 3.14 x 25 = 78.5 square units.
A semi circle is half a full circle
Area of the semi circle = 78.5/2 = 39.25 square units.
