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:

The distance across the circle. The length of any chord passing through the center. It is twice the radius.
Diameter$$=$$ $2·\mathrm{Radius}$ $$=$$ $$2r$$

$\mathrm{Radius}=\frac{\mathrm{Diameter}}{2}$
Area of the Circle $$=$$ $\mathrm{\pi }{r}^{2}$ $$sq. units$$
Here $$r$$ is the radius of the circle.
The value of the $\mathrm{\pi }$ [pi] can be either $$22/7$$ or $$3.14$$.