This means that, using Pythagoras’ theorem, the equation of a circle with radius r and centre (0, 0) is given by the formula \(x^2 + y^2 = r^2\). Find the equation of a circle with radius 3 ...

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.