\documentclass[12pt, a4paper]{book}
% Paquetes
\usepackage[spanish]{babel}
\usepackage{graphicx}
\usepackage{geometry}
\usepackage{csquotes}
\usepackage{parskip}
\usepackage{fancyhdr}
\usepackage[style = apa, backend = biber]{biblatex}
\usepackage{setspace}
% Recursos bibliográficos
\addbibresource{ref.bib}
% Formato de página
\geometry{a4paper, left=3.5cm,right=2.54cm,top=2.54cm,bottom=2.54cm}
\setlength{\parindent}{2em}
\pagestyle{fancy}
\fancyhf{} % clear all header and footer fields
\renewcommand{\headrulewidth}{0pt} % no line in header area
\fancyfoot[R]{\thepage} % page number in the right footer
\fancypagestyle{plain}{%
\fancyhf{} % clear all header and footer fields
\fancyfoot[R]{\thepage} % page number in the right footer
}
% Tesis
\begin{document}
\input{frontmatter/frontmatter}
\mainmatter
\renewcommand{\baselinestretch}{1.5}
\pagenumbering{arabic}
\input{mainmatter/intro}
\input{mainmatter/problematica}
\input{mainmatter/rev literatura}
\input{mainmatter/variables}
\printbibliography
\end{document}
