The formula for circumference in terms of a circle’s diameter is C = πd. The diameter is twice the radius (d = 2r). So, replace d with 2r and rewrite the formula for circumference in terms of radius r:
"Formula of a circle" is too vague to be meaningful. Perhaps you meant, "Formula for the area of a circle in terms of its circumference."
The area of a circle in terms of its radius is A = πr^2. To put this formula to use, we have to know the radius of the circle. The circumference of a circle in terms of its radius is C = 2πr, so a formula for the radius is r = C / (2π).
Now let's find a formula for the area of a circle in terms of its circumference:
C C^2
A = πr^2 = π { ---------------- }^2 = ------------
