# Get an underbrace under a matrix

Hi I have a matrix that i want an underbrace under. But I do not manage to do it.

\underbrace{
\begin{bmatrix}
&2(r_1^T - r_2^T)  &2(t_2-t_1) \\
&2(r_1^T - r_3^T)  &2(t_3-t_1) \\
&2(r_1^T - r_4^T)  &2(t_4-t_1) \\
&2(r_1^T - r_5^T)  &2(t_5-t_1) \\
&2(r_1^T - r_6^T)  &2(t_6-t_1) \\
&2(r_1^T - r_7^T)  &2(t_7-t_1) \\
&2(r_1^T - r_8^T)  &2(t_8-t_1)
\end{bmatrix}_{\text{$=A$}}


You are missing the closing brace of \underbrace{...}. Also, you should not start each line of the matrix with the column divider &, as that will give an empty column at the start.

I have added \renewcommand\arraystretch{1.25} to increase the line spacing of the matrix.

There is no reason to use \text{$...$} in the underbrace text, as that is already typeset in math mode.

\documentclass[preview,border=5pt]{standalone}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\renewcommand\arraystretch{1.25}
\underbrace{
\begin{bmatrix}
2(r_1^T - r_2^T) & 2(t_2-t_1) \\
2(r_1^T - r_3^T) & 2(t_3-t_1) \\
2(r_1^T - r_4^T) & 2(t_4-t_1) \\
2(r_1^T - r_5^T) & 2(t_5-t_1) \\
2(r_1^T - r_6^T) & 2(t_6-t_1) \\
2(r_1^T - r_7^T) & 2(t_7-t_1) \\
2(r_1^T - r_8^T) & 2(t_8-t_1)
\end{bmatrix}}_{=A}
\end{equation*}

\end{document}


• I have accepted the answer. The arraystretch was helpfull. Kind of a brain fart to forget the } – Olba12 Mar 2 '16 at 9:19