\documentclass[10pt]{article} %% What type of document you're writing.
\usepackage{amsmath,amsfonts,amssymb} %% AMS mathematics macros
%% Title Information.
\title{Aumento Porcentual a Precio Aumentado \& Disminuci\'on Porcentual a Precio Disminuido}
\author{Jose Perez}
%% \date{2 July 2004} %% By default, LaTeX uses the current date
%%%%% The Document
\begin{document}
\maketitle
\section{Aumento Porcentual a Precio Aumentado}
$$
\%_A=\frac{P_A-P_i}{P_i}\times 100 \\
$$
$$
P_i\times(\%_A)=(P_A-P_i)\times 100 \\
$$
$$
P_i\times(\%_A)=P_A\times 100-P_i\times 100 \\
$$
$$
P_i\times(\%_A)+P_i\times 100=P_A\times 100 \\
$$
$$
P_i(\%_A+100)=P_A\times 100 \\
$$
$$
P_A\times 100=P_i(\%_A+100) \\
$$
$$
P_A=\frac{P_i(\%_A+100)}{100} \\
$$
$$
P_A=P_i\left (\frac{\%_A+100}{100} \right ) \\
$$
$$
P_A=P_i\left (\frac{\%_A}{100}+\frac{100}{100} \right ) \\
$$
$$
P_A=P_i\left (\frac{\%_A}{100}+1 \right ) \\
$$
$$
P_A=P_i\left (1+\frac{\%_A}{100} \right ) \\
$$
\newpage
\section{Disminuci\'on Porcentual a Precio Disminuido}
$$
\%_D=\frac{P_i-P_D}{P_i}\times 100 \\
$$
$$
P_i\times(\%_D)=(P_i-P_D)\times 100 \\
$$
$$
P_i\times(\%_D)=P_i\times 100-P_D\times 100 \\
$$
$$
P_i\times(\%_D)-P_i\times 100=-P_D\times 100 \\
$$
$$
P_i(\%_D-100)=-P_D\times 100 \\
$$
$$
-P_D\times 100=P_i(\%_D-100) \\
$$
$$
P_D=\frac{P_i(\%_D-100)}{-100} \\
$$
$$
P_D=P_i\left (\frac{\%_D-100}{-100} \right ) \\
$$
$$
P_D=P_i\left (\frac{\%_D}{-100}-\frac{100}{-100} \right ) \\
$$
$$
P_D=P_i\left (-\frac{\%_D}{100}+1 \right ) \\
$$
$$
P_D=P_i\left (1-\frac{\%_D}{100} \right )
$$
\end{document}
