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Radius
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In geometry, the radius of a circle or sphere, written as [math]r[/math],^{[1]} 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.^{[2]} That is, when r is the radius and d is the diameter:^{[3]}
r = d ÷ 2
d = 2 x r = r + r
The relationship between the radius [math]{\displaystyle r}[/math] and the circumference [math]{\displaystyle c}[/math] of a circle is [math]~c = 2\pi r.[/math]
The area [math]{\displaystyle A}[/math] of a circle of radius [math]{\displaystyle r}[/math] is [math]{\displaystyle ~A = \pi r^2.}[/math]
Related pages
References
 ↑ "List of Geometry and Trigonometry Symbols" (in enUS). 20200417. https://mathvault.ca/hub/highermath/mathsymbols/geometrytrigonometrysymbols/.
 ↑ Weisstein, Eric W.. "Radius" (in en). https://mathworld.wolfram.com/Radius.html#:~:text=The%20distance%20from%20the%20center,equal%20to%20half%20the%20diameter..
 ↑ "Radius, diameter, & circumference  Circles (article)" (in en). https://www.khanacademy.org/math/ccseventhgrademath/cc7thgeometry/cc7thareacircumference/a/radiusdiametercircumference.
Other websites
 Definition and properties of the radius of a circle with interactive applet
