

$$C = 2 \pi r$$
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 $$\pi \simeq 3.14$$