Actually the circle is the one kind of line means the type of line and also the set of points which are equidistant to any fix point of plane. This constant distance called the Radius of circle and that fix point are called the Center of circle.
The radius of a circle is the length of the line from the center to any point circle on its edge. The radius of circle is always constant. The plural form of the radius is - radii (pronounced "ray-dee-eye"). Sometimes the word 'radius' is used to refer to the line itself. In that sense you may see "draw a radius of the circle". In the more recent sense, it is the length of the line, and so is referred to as "the radius of the circle is 2 centimeters"
(1) DIAMETER
when D = diameter of the circle
Radius is simply half the diameter.
(2) CIRCUMFERENCE
when C = circumference of the circle
D = diameter of the circle
The ratio of a circle's circumference to its diameter is π (pi), an irrational constant approximately equal to 3.141592654. Thus the length of the circumference C is related to the radius r and diameter d.
(3) AREA ENCLOSED
when A = the area of the circle
As proved by Archimedes, the area enclosed by a circle is equal to that of a triangle whose base has the length of the circle's circumference and whose height equals the circle's radius,which comes to π multiplied by the radius squared:
Equivalently, denoting diameter by d,
that is, approximately 79 percent of the circumscribing square (whose side is of length d).
The circle is the plane curve enclosing the maximum area for a given arc length. This relates the circle to a problem in the calculus of variations, namely the isoperimetric inequality.
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