\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
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\begin{document}
\begin{equation*}
  \arg{G(j\omega_0)}  \approx
    \frac{\pi}{2} \frac{d\log{|G(j\omega)|}} {d\log{\omega}}.
\end{equation*}
\end{document}