• a circle that passes through all the vertices of a polygon.
• the centre of a circumcircle is called the circumcentre
and the radius is called the circumradius.
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• all regular polygons, triangles, rectangles and the
isosceles trapezium (UK)/trapezoid (US)
may be circumscribed with a circle.
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• Other irregular polygons are not usually considered to have a circumcircle.
However certain irregular shapes may qualify.
