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\documentclass[12pt,a4paper,oneside] {article}
\usepackage[koi8-r]{inputenc}
\usepackage[english,russian]{babel}
\usepackage{indentfirst}
\usepackage{misccorr}
\usepackage{amsmath}
\begin{document}
\section{Basic Formulas}
\begin{equation}
D = (n - n0)(\frac{1}{R1} - \frac{1}{R2})
\end{equation}
\end{document}