how can I center the first equation Max $Z = 0.15x_{1} + 0.12x_{2} + 0.1x_{3} + 0.05x_{4} + 0.08x_{5}$ and the text "s.a" with the other equations below ?
\documentclass{article}
\usepackage[margin=1in]{geometry}
\usepackage[spanish,es-tabla]{babel} % espanol
\decimalpoint
\usepackage[utf8]{inputenc}
\usepackage{graphicx} % graficos
\usepackage{amsmath}
\begin{document}
Luego el modelo matemático queda planteado como se muestra a continuación:\\
Max $Z = 0.15x_{1} + 0.12x_{2} + 0.1x_{3} + 0.05x_{4} + 0.08x_{5}$
s.a
\begin{equation}
x_{1} + x_{2} + x_{3} + x_{4} + x_{5} \leq 1.000.000
\label{eqn:p1-r1}
\end{equation}
\begin{equation}
x_{1} + x_{2} \leq 0.3(x_{1} + x_{2} + x_{3} + x_{4} + x_{5})
\label{eqn:p1-r2}
\end{equation}
\begin{equation}
x_{4} \geq x_{5}
\label{eqn:p1-r3}
\end{equation}
\begin{equation}
x_{3} + x_{4} \leq 0.5(x_{1} + x_{2} + x_{3} + x_{4} + x_{5})
\label{eqn:p1-r4}
\end{equation}
\begin{equation}
x_{4} \geq 0.25(x_{1} + x_{2} + x_{3} + x_{4} + x_{5})
\label{eqn:p1-r5}
\end{equation}
\begin{itemize}
\item Resuelva el problema mediante el software de optimización OPL –CPLEX, y entregue la Solución y Valor
Optimo.\\
\end{itemize}
\end{document}


MWE. Though not THE way to typeset these type of equation. You can do it by usingdisplay-mathstyle. Replace$...$by\[...\]. – Raaja_is_at_topanswers.xyz Nov 23 '18 at 19:55amsmath, you might use thegatherenvironment (with\\to separate lines) instead of entering each line as a separateequation. and don't leave a blank line before any single or multi-line equation block. the blank line will not prevent a page break (which is not desirable in such a location), and the use of a multi-line environment likegatherwill allow better control over page breaks between lines. – barbara beeton Nov 23 '18 at 22:59