Hello everyone!
I have some questions about equatuons:
1)  I want to get from rst source latex code like this: How can I do this? Would it be ok just to use two math:: directives each with :label: option? The question is on alignment.
\begin{eqnarray}
m_1\ddot{x}_1+(k_1+k_2)x_1-k_2x_2 & = & u_1, \label{spmass1}\\
m_2\ddot{x}_2-k_2x_1+(k_1+k_2)x_2 & = & u_2 . \label{spmass2}
\end{eqnarray}

2) Is it possible to get labels for equations like these:
\begin{equation}
\dot{x}(t) = f(t, x, u),
\label{ctds1}
\end{equation}

\begin{equation}
x(t+1) = f(t, x, u),
\tag*{(\ref{ctds1}d)}
\label{dtds1}
\end{equation} ?
I mean I want labels to look like (1) and (1d), for example. 

3)  What about referencing equation from another file (chapter)? Is it possible?

Lukashevichus Ksenia.