Radius
From Exampleproblems
- For other senses of this word, see radius (disambiguation).
In classical geometry, a radius of a circle or sphere is any line segment from its center to its boundary. By extension, the radius of a circle or sphere is the length of any such segment. The radius is half the diameter. In science and engineering the radius of a curved surface is often called the radius of curvature, especially when there are other "radii" in the problem that could be confused with it.
More generally — in geometry, engineering, graph theory, and many other contexts — the radius of something (e.g., a cylinder, a polygon, a graph, or a mechanical part) is the distance from its center or axis of symmetry to its outermost points. In this case, the radius may be more than half the diameter.
The relationship between the radius and the circumference is 
.
