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+\documentclass[12pt]{article}
+
+\begin{document}
+
+$$
+  E = \frac{1}{2} K \cos^2{\left(\chi - \chi_0\right)}
+$$
+
+\end{document}
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+\documentclass[12pt]{article}
+
+\begin{document}
+
+\begin{eqnarray*}
+   E & = &\frac{1}{6} K \left(\Delta_{ijl} + \Delta_{ijk} + \Delta_{kjl} \right)^6 \\
+   \Delta_{ijl} & = & \cos{\theta_{ijl} - \cos{\theta_0}} \\
+   \Delta_{ijk} & = & \cos{\theta_{ijk} - \cos{\theta_0}} \\
+   \Delta_{kjl} & = & \cos{\theta_{kjl} - \cos{\theta_0}}
+\end{eqnarray*}
+
+\end{document}