Let there be a circle. If the radius of this circle we do not know, you can find it in several ways:
- to know the radius of a simple measurement of the radius of its circumference,
- if you know the circumference of this circle, its radius can be calculated according to the formula R = L/2P, where L is the length of the circumference,
- can you describe a square about a circle, then its radius is equal to half the side of the square.
From the school course of geometry known theorem - area of a circle is equal to half of the length of work limiting its circumference to the radius.
S = P*R*R