

$$C = 2 \pi r$$
Circumference
$$r = \frac{C}{2 \pi }$$
Radius
$$A = \pi {r}^2$$
Area
$$r = \sqrt{\frac{A}{\pi}}$$
Radius
$$d = 2 r$$
Diameter
$$r = \frac{d}{2}$$
Radius
Definition
A circle is the set of all points in a plane that are equidistant from a given point called the center of the circle.
Properties
 Radius: segment that extends from the center of the circle to any point of the circumference
 Diameter: segment passing through the center that extends two points of the circumference. It's double the radius
 Pi (symbol $\pi$) constant value approximated as $$\pi \simeq 3.14$$
Circle Formulas
Data  Formula 

Circumference  C = 2 π r 
Area  A = π r^{2} 
Diameter  d = 2 × r 
Radius  r = C / (2 π) 
Radius  r = √( A / π ) 
Radius  r = d / 2 