The area of a circle is the size of the 2-dimensional space inside the circle's
closed curved boundary.
The area can be calculated in terms of known linear measurements of the circle:
-- Area = (π) x (radius)²
-- Area = (π/4) x (diameter)²
-- Area = (1/2) x (circumference) x (radius)
-- Area = (1/4) x (circumference) x (diameter)
Any of these formulas will give you the area. The one you decide to use
just depends on what you already know about the circle.