Radius (plural: radii) is a part of an ellipse or circle.
The radius of a circle is the distance between its center and a point in it, which is half its diameter. For a circle defined as ( x − a ) 2 + ( y − b ) 2 = r 2 {\displaystyle (x-a)^2+(y-b)^2=r^2} , r {\displaystyle r} is its radius.
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is its radius.