From center & radius or general form: equation, center, radius, graph.
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Why it works
Completing the square turns x²+Dx into (x+D/2)² − (D/2)², which exposes the center (−D/2, −E/2) and radius r = √((D/2)²+(E/2)²−F). Every circle is just the set of points at distance r from its center — the distance formula in disguise.
