Some useful Preambles

\usepackage[hypcap]{caption}
\usepackage{amssymb}
\usepackage{mathtools}
\usepackage{times}
\usepackage{fancyhdr}
\usepackage[pdftex]{graphics}
\usepackage{graphicx}
\usepackage{latexsym}
\usepackage{amsmath,amsthm}

\usepackage{ifthen}
\usepackage{xifthen}


\usepackage{cancel}
\usepackage{enumerate}
\usepackage{geometry}


\usepackage{float}

\usepackage{scrextend}

\usepackage{xcolor}
\usepackage{graphicx}
\usepackage{ifthen}
\usepackage{xifthen}

\usepackage{amssymb,amsmath,amsthm}

\usepackage{pgfplots}


\usepackage{pgf,pgffor}


\usepackage{tikz}
\usepackage{tkz-euclide}
%%!
%%! For older versions of package tkz-euclide
%%! you may need uncomment the following line:
%   \usetkzobj{all}
%%!

\usetikzlibrary
    {
      shapes.geometric,
      arrows,
      calc,
      intersections,
      arrows.meta,
      decorations.markings,
      positioning,
      math,
      angles,
      quotes,
      patterns,3d,
      backgrounds,
      fillbetween
    }
    
%%Usage: \gettikzxy{(P)}{\px}{\py}
\makeatletter
\newcommand{\gettikzxy}[3]{%
  \tikz@scan@one@point\pgfutil@firstofone#1\relax
  \edef#2{\the\pgf@x}%
  \edef#3{\the\pgf@y}%
  \tikzmath{\#2=#2/28.3465;};
  \tikzmath{\#3=#3/28.3465;};
}
\makeatother
\pgfplotsset{compat=1.14}

\tikzstyle{arrow} = [thick,-,>=stealth]
\tikzstyle{myarrow} = [thick,->,>=stealth]

\geometry{
  a4paper,
  paperwidth=21cm,
  paperheight=35cm,
  headheight=2.5cm,
  total=17cm,
  left=3cm,
  right=3cm,
  top=3cm,
  headsep=0.5cm,
  hoffset=-1.5cm,
  bottom=3cm,
}

\setlength{\parindent}{0cm}

\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[brazilian]{babel}

\DeclareMathOperator{\Arctan}{Arctan}
\DeclareMathOperator{\Arcsin}{Arcsin}
\DeclareMathOperator{\Artanh}{Artanh}
\DeclareMathOperator{\Real}{Re}
\DeclareMathOperator{\Image}{Im}
\DeclareMathOperator{\Det}{Det}
\DeclareMathOperator{\Trace}{Tr}
\DeclareMathOperator{\Ger}{span}
\DeclareMathOperator{\Span}{span}
\DeclareMathOperator{\Dim}{dim}

\newcommand{\Vector}[1]{\underline{\mathbf{#1}}}
\newcommand{\Matrix}[1]{\underline{\underline{{\mathbf{#1}}}}}

\newcommand{\Result}[1]{\underline{\underline{~~#1~~}}}

\newtheorem{Axiom}{Axioma}
\newtheorem{Theorem}{Teorema}[section]
\newtheorem
    {Definition}
    [Theorem]
    {Definição}
\newtheorem
    {Proposition}
    [Theorem]
    {Proposição}
\newtheorem
    {Corollary}
    [Theorem]
    {Corolário}
\newtheorem
    {Lemma}
    [Theorem]
    {Lema}

\newtheorem
    {Comment}
    [Theorem]
    {Comment}

\usepackage{listings}
\lstset{ 
  basicstyle=\footnotesize,        % the size of the fonts that are used for the code
  breakatwhitespace=false,         % sets if automatic breaks should only happen at whitespace
  %keywordstyle=\color{blue},       % keyword style
  language=Tex,                    % the language of the code
}