geometry package can't change margin in article document

Question

I am using article document class and I want to narrow down the margin.

\documentclass{article}
\usepackage{enumitem,amssymb}
\usepackage{amsmath}
\usepackage[margin=50mm]{geometry}
\begin{document}
\everymath{\displaystyle}
\section{Trigonometric and hyperbolic equations}
\begin{itemize}
  \item ( 1 + \tan^2 x = \sec^2 x , \quad 1 + \cot^2 x = \csc^2 x )
\end{itemize}
\end{document}

Questions:

1. How to correctly narrow down margin in article document?
2. Why the heck does article generate such a giant ugly margin? What's the reason and logic?