Radius

In geometry, the radius of a circle or sphere, written as , is the shortest connection between the center and the boundary. It is either the distance from the center to the perimeter (in the case of a circle), or the distance from the center to the surface (in the case of a sphere). It is half of the diameter. That is, when r is the radius and d is the diameter:

r = d ÷ 2

d = 2 x r = r + r

The relationship between the radius  and the circumference  of a circle is 

The area  of a circle of radius 
 is 

Related pages
 Area
 Covalent radius
 Diameter
 Radian

 References 

 Other websites 
 Definition and properties of the radius of a circle with interactive applet

Category:Geometry