A circle is the set of points equidistant from a point C(h,k) called the center. The fixed distance r from the center to any point on the circle is called the radius. The standard equation of a circle with center C(h,k) and radius r is as follows.
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(x-h){^2} + (y-k){^2} = r{^2}
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Reference. http://www.analyzemath.com/CircleEq/Tutorials.html