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f / r² + a   =   n + k

                                    Subtract  a  from both sides of the equation.

f / r²   =   n + k - a

                                    Multiply both sides of the equation by  r² .

f   =   r²(n + k - a)

                                    Divide both sides of the equation by  (n + k - a)

f / (n + k - a)  =  r²

                                    Take the ± square root of both sides.

±√[ f / (n + k - a) ]  =  r

 

r  =  ±√[ f / (n + k - a) ]