[Tex/LaTex] Parenthesis don’t appear in beamer

beamerequationsparentheses

I'd like to show parenthesis in equation environment for beamer presentation. I don't know because it not appear. 

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[portuguese]{babel}
\usepackage{mathtools}

\usepackage{upgreek}
\usepackage{amsmath}
\usepackage{bbold}
\usepackage[subnum]{cases}
\usepackage{MnSymbol}
\usepackage{lmodern} % required to show the problem

\usepackage{ragged2e}
\usepackage{etoolbox}


\apptocmd{\frame}{}{\justifying}{} 
\newcommand{\bs}{\mathbf}

\usetheme{Madrid}

\begin{document}
\section{Introdução}
%\subsection{First Subsection}
\begin{frame}{Unified Particle Swarm Optimizer - UPSO}
    \justifying
    \setbeamercolor{block title}{fg=white,bg=red!75!black}
    \begin{block}{PSO tradicional}
        PSO tradicional foi proposto por Eberhart, R. C. and Kennedy, J. (1995) e apresenta as seguintes equações:
        \begin{equation*}
            \begin{aligned}
                v_{i+1} = \omega v_i + r_1c_1 (\bs{lbest_i} - \bs{x_i}) + r_2c_2 (\bs{gbest_i} - \bs{x_i})\\
                x_{i+1} = x_i + v_{i}\\
            \end{aligned}
        \end{equation*}

    \end{block}
\end{frame}
\end{document}

Best Answer

When you change the order of the packages lmodern and MnSymbol to

\usepackage{lmodern}
\usepackage{MnSymbol}

than you get the parentheses again.

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