Пример использования стиля eqalign.sty

\documentstyle[eqalign]{article}
\def\diff#1#2{\frac{\partial#1}{\partial#2}}
\begin{document}

\begin{equation}
\eqalign{
        \diff{\rho}{t} + \vec{v}\nabla\rho & = 0\,, \cr
        \diff{\vec{v}}{t} + (\vec{v}\vec{\nabla})\vec{v} 
	                                   & = -\frac{1}{\rho}\nabla P\,, \cr
}
\end{equation}

and

\begin{equation}
\eqalignleft{
        v \propto r^2\,, \quad & \hbox{\rm for } r < R_0\,, \cr
        v = 0\,,               & \hbox{\rm for } r > R_0\,. \cr
}
\end{equation}

\end{document}