and \(\sqrt {{g^2} + {f^2} - c}\) is the radius. Notice that for the circle to exist, \({g^2} + {f^2} - c\textgreater0\). Look at the following worked examples.

The tangent is perpendicular to the radius which joins the centre of the circle to the point P. As the tangent is a straight line, the equation of the tangent will be of the form \(y = mx + c\).