$$2tan \alpha_n \left({x_ {n1} +x_{n2} \over z_1 + z_2}\right) + inv \alpha_s$$
$${z_1 + z_2 \over 2cos \beta_ 0} \left({cos \alpha_s \over cos \alpha_ {bs}} - 1\right)\ $$
$$\left({z_1 + z_2 \over 2cos \beta_ 0} + y\right) m_n$$
$$tan^{-1}\left({tan \alpha_n \over cos \beta_ 0} \right)$$