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In geometry, the radius of a circle or sphere, written as \displaystyle{ r }, 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
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