Completing the square is used to find the radius and center of circles. The standard form for a circle is \(ax^2 + by^2 + cx + dy + e =0\). To find the radius and center of the circle the equation will need to be in the form \((x-h)^2 + (y-k)^2 = r^2\) where center is (\(h,k\)) and the radius is \(r\).
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Completing the square is also used to find the center and vertices of an ellipse.