By Staff Writer Last Updated Apr 15 2020 31728 PM ET. The radius of the circle is the length of a straight line stretching from the center of the circle to the line of circumference. A p 4 D2.
Area of a circle diameter.
C 2 p r. Area of a circle radius. A p r2. The diameter of a circle is the length of a straight line drawn between two points on a circle where the line also passes through the centre of a.