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# Radius

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In geometry, the **radius** of a circle or sphere is the shortest connection between the center and the boundary. It is half of the diameter.

r= d ÷ 2

d= 2 x r = d= r + r

r= Radius d= Diameter

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]

## Other pages

## Other websites

- Definition and properties of the radius of a circle with interactive applet