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\begin{document}

Consider the equation
\begin{equation}\label{eq:myequation}
\begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}x = \begin{pmatrix} 1 \\ -1 \end{pmatrix}.
\end{equation}
It is easy to solve \eqref{eq:myequation}.
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