Let L be the length of the circumference, π a constant, the value of which is constant (π=3.14). Then, to determine the radius given the circumference, you need to use the formula:
R = L/2π
Example: length of a circle is 20 cm. Then the radius of this circle R = 20/2*3.14 = 3.18 cm
Let the famous S - area of a circle. Then, knowing the formula for finding the area of a circle (S = πR2), it's easy to withdraw and another to determine the radius of a circle:
R = √(S/π)
Example: area of a circle is 100 cm2, then its radius is R = √(100/3.14) = 5.64 cm
If the circumference length of the diameter (a line segment which connects two opposite points of the circle, passing through its center), the task of finding the radius is to divide the length of the diameter of the circle by 2.