Math Homework. Do It Faster, Learn It Better.
Radius
A radius of a circle (or sphere) is a line segment with one endpoint at the center, and the other endpoint at the boundary. The word is also used to mean the length of such a segment.
All the radii of a particular circle (or sphere) are equal in length. The length of the radius of a circle is equal to
,
where is the circumference of the circle, and is pi , an irrational number approximately equal to .
The radius is also used in the calculation of the area of a circle. The formula is
.
See also diameter .