# multiple equations with same number

I can't seem to make this work. I want the matrix and the second equation with the same equation number.

\begin{aligned} [{\bf Z}] &=\begin{bmatrix} R_1+j\omega L_1 & -j\omega M \\ j\omega M & -R_2-j\omega L_2 \end{bmatrix} \\ k &=\frac{M}{\sqrt{L_1L_2}} \end{aligned}

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If you're referring to {\bf Z} as a "vector" or "matrix", then consider reading How can I get bold math symbols? –  Werner Jun 10 '14 at 4:24

This is actually quite a subtle thing that has come up in a few different varieties on the site.

The aligned environment takes an optional argument; in your use case, you have been unlucky enough to select a combination that makes LaTeX think that you are supplying the aligned environment with the argument {\bf Z}, when actually you intend to skip the optional argument altogether, and begin your content with [{\bf Z}].

So, we have to tell LaTeX not to look for an optional argument, which we can do using \relax.

% arara: pdflatex
\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{aligned}\relax [{\bf Z}] &= \begin{bmatrix} R_1+j\omega L_1 & -j\omega M \\ j\omega M & -R_2-j\omega L_2 \end{bmatrix} \\ k &=\frac{M}{\sqrt{L_1L_2}} \end{aligned}
\end{document}


Alternatively, you can 'protect' [{\bf Z}] by using {[\bf Z]}, which is demonstrated in "[" as first character in table row. However, as discussed in the comments, and in What is the difference between \relax and {}?, \relax is the safer option.

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