U_{op}=\frac{1}{2}k (r-l)^2=\frac{1}{2}k (\sqrt{2 x^2-2Cx+C^2}-l)^2=\frac{1}{2}k ( 2 x^2-2Cx+C^2 + l^2 - 2l\sqrt{2 x^2-2Cx+C^2})