 Choose coplanar unit vectors \(\mathbf{a},\) \(\mathbf{a}',\) \(\mathbf{b},\) \(\mathbf{b}'\) such that \(\theta_{\mathbf{b}'} - \theta_{\mathbf{a}}\, =\) \(\theta_{\mathbf{a}} - \theta_{\mathbf{b}}\, =\) \(\theta_{\mathbf{b}} - \theta_{\mathbf{a}'} = \phi\), and therefore, \(\theta_{\mathbf{b}'} - \theta_{\mathbf{a}'} = 3\phi\). This choice yields 