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+\documentstyle[12pt]{article}
+
+\begin{document}
+
+$$
+E = A \exp \left(\frac{\sigma - r}{\rho} \right) - 
+    \frac{C}{r^6} + \frac{D}{r^8} + \frac{kq_i q_j}{\epsilon r}
+$$
+
+\end{document}