The formula for area of a circle is r^2*pi=a. If you know that the radius is three, then you can find the area of at 9*pi, or 9*3.141592653589793238462643383279502884197169399375105820974944592307816406286208... This can be simplified to about 27. The formula for circumference is diameter times pi. The diameter is the same as two radii, so the formula can also be two times radius times pi. If the radius is three, then the diameter is six. Six times pi can simplify to about 18.