Теория:

A closed line consisting of all points on the plane that are equidistant from a given point on the plane is called a circle.
 
Diameter of Circle:
  
Diameter.svg
The distance across the circle. The length of any chord passing through the center. It is twice the radius.
Diameter\(=\) 2 ·Radius \(=\) \(2r\)
 
  
Radius of circle:
 
radius.svg
 
The radius is the distance from the centre to any point on the circle.
Radius is also known as half  of the diameter.
Radius=Diameter2
 
  
Area of the Circle:
The area of a circle is the number of square units inside that circle.
Area of the Circle \(=\) πr2 \(sq. units\)
Area.svg
 
Here \(r\) is the radius of the circle.
The value of the π [pi] can be either \(22/7\) or \(3.14\).