Some other shapes besides circles that also can have radii are regular polygons like a pentagon or hexagon. The definition of the radius for a regular polygon is that it is the distance from the center point of the polygon to one of its vertices. Regular polygons are figures that have side lengths that are all congruent.
There are formulas to find the radius if one is given either the side length or the apothem of the regular polygon. The apothem, or inradius, is the distance from its center point to any side. The formula for finding the radius of a regular polygon if the side length is known is given as radius = side length/ 2 sin (180/n), where n is the polygon's number of sides. This formula is useful for problems that involve an inscribed circle in a regular polygon.
For a circle, the radius is the distance from its center point to any point located on the circle edge. The diameter is twice the radius.