In geometry, the radius of a circle or sphere is the shortest connection between the center and the boundary. It is half of the diameter.

r= d ÷ 2

d= 2 x r = d= r + r

The relationship between the radius ${\displaystyle r}$ and the circumference ${\displaystyle c}$ of a circle is $~c = 2\pi r.$
The area ${\displaystyle A}$ of a circle of radius ${\displaystyle r}$ is ${\displaystyle ~A = \pi r^2.}$