\begin{equation} \ \ \ \ \ \ \ \ \ \ \ \times\left(\left(k_{1}X_{1}+k_{2}X_{2}\right)+2k_{\text{bf}}-\left(\varphi_{1}k_{1}X_{1}+\varphi_{2}k_{2}X_{2}\right)-2\varphi_{\text{hnf}}k_{\text{bf}}\right)^{-1}\nonumber \\ \end{equation}