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\begin{document}

\begin{eqnarray*}
 c = l_z - 0.5(l_y+l_x) \\
 b = l_y - l_x \\
 k = \frac{3}{2} \frac{l_x^2+l_y^2+l_z^2}{(l_x+l_y+l_z)^2} - \frac{1}{2} 
\end{eqnarray*}

\end{document}

