In addition to the radius you need to know either the degree of the central angle or the length of the arc. Area of a circle diameter. The formula to find a circles area p radius 2 usually expressed as p r 2 where r is the radius of a circle.
The radius is the length from the center of a.
Area pr 2. Or when you know the Circumference. A diameter of a circle is defined as a straight line passing from one side to another side through the center point. Area of a circle in terms of circumference.