Radius

In classical geometry, a radius (pl.: radii or radiuses) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is the line segment or distance from its center to any of its vertices. The name comes from the Latin radius, meaning ray but also the spoke of a chariot wheel. The typical abbreviation and mathematical symbol for radius is R or r. By extension, the diameter (denoted D or d) is defined as twice the radius: d ≐ 2 r ⇒ r = 1 2 d {\displaystyle d\doteq 2r\quad \Rightarrow \quad r={\tfrac {1}{2}}d} If an object does not have a center, the term may refer to its circumradius, th (Wikipedia)