\documentclass{article}

\begin{document}

\title{My Second Document}
\author{J. Doe}
\maketitle

\section{Quadratic equations}
\label{sec:quad}

The general quadratic equation
\begin{equation}
  a x^2 + b x + c = 0  \label{eq:quad}
\end{equation}
has two solutions given by
\begin{equation}
  x_{1,2} = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}  \label{eq:quadsol}
\end{equation}

\section{Numerical solution}
\label{sec:numsol}

In section \ref{sec:quad}, we gave the solution of the quadratic
equation.  This solution can be put in a computer program as is, but
is then likely to suffer from round-off error problems.

The better way to do it is to find only one solution, call it $x_1$,
from the given formulae (\ref{eq:quadsol}), selecting $x_1$ as {\em
  the solution for which the two term in the top of the ratio have the
  same sign.}  Then compute the other solution from
\begin{equation}
  x_2 = \frac{c}{a x_1}  \label{eq:altsol}
\end{equation}

\end{document}
