Equations with text and math interpersed

This is related to the question I asked here:

Equations with only text in them

Basically I want to have an equation with a lot of text and some math in them. I don't want to use \mbox because there is a lot of text and it may start going out of the line. In the linked question a solution is provided using \parbox but I want to use math also in the middle.

For example:

\begin{equation}\label{key}
The product of two complex numbers z_1,z_2\in\mathbb{C} is given by the vector whose
angle is the sum of the angles of the two complex numbers,
i.e. arz(z_1z_2)=arg(z_1)+arg(z_2) and whose magnitude is the product of the
magnitudes of the two complex numbers, i.e. |z_1z_2|=|z_1||z_2|
\end{equation}

How do I accomplish this?

• Wouldn't a theorem-like definition environment be more appropriate than an equation environment, here? (Hint: yes) – jub0bs Aug 12 '14 at 1:46
• @Jubobs: How do I do that? – Shahab Aug 12 '14 at 2:01
• @Jubobs: Can numbering be accomplished in such an environment in a manner similar to equations? – Shahab Aug 12 '14 at 2:12

May be you can do this.

\documentclass{article}
\usepackage{amsmath}   %%% habitual addition
\usepackage{amsfonts}
\usepackage{calc}
\newlength{\mylen}
\setlength{\mylen}{\widthof{(99)}}
\begin{document}
\begin{equation}\label{key}
\left.\begin{minipage}{\dimexpr\textwidth-2\mylen\relax}
The product of two complex numbers $z_1$, $z_2\in\mathbb{C}$ is given by the vector whose
angle is the sum of the angles of the two complex numbers,
i.e. $\arg(z_1z_2)=\arg(z_1)+\arg(z_2)$ and whose magnitude is the product of the
magnitudes of the two complex numbers, i.e. $|z_1z_2|=|z_1||z_2|$
\end{minipage}
\right\}
\end{equation}
\end{document} The same can be done with \parbox too.

The "equation" in question contains not one but two entities that many people would interpret to be equations; moreover, both the text and the equations are very important. I therefore think you (and likely your paper's readers as well) would be better off if you chose a theorem-like environment to typeset the material, rather than insist on it being "just an equation".

This environment -- let's call it Statement for the sake of specificity -- can be cross-referenced easily. If you wish, you can use the equation counter to number the statement environments. An example of this usage is given below. \documentclass{article}
\usepackage{amssymb,ntheorem}
\usepackage[noabbrev]{cleveref} % for "clever" cross-references
\theorembodyfont{\upshape} % choose \itshape if you prefer
\newtheorem{statement}[equation]{Statement}
\begin{document}
An equation:
\begin{equation}\label{eq:pyth}
a^2+b^2=c^2
\end{equation}

\begin{statement}\label{complex-prod}
The product of two complex numbers $z_1,z_2\in\mathbb{C}$ is given by the vector whose
angle is the sum of the angles of the two complex numbers,
i.e.\ $\arg(z_1z_2)=\arg(z_1)+\arg(z_2)$ and whose magnitude is the product of the
magnitudes of the two complex numbers, i.e.\ $|z_1z_2|=|z_1||z_2|$.
\end{statement}

Another equation:
\begin{equation}\label{eq:euler}
\exp(i\pi)-1=0
\end{equation}

As was shown in \cref{eq:euler,complex-prod,eq:pyth}, \ldots

\end{document}