How to have a matrix aligned with the rest of an equation?

Question

When I include a matrix in an equation, the first part of the equation is separated off to the left of the page, while the matrix is spaced away in its own centred area on its own line. Here is what I am currently doing:

\[
$M^{-T} = \frac{1}{|det(M)|}$  \begin{bmatrix}
a_{22} & - a_{21} \\
-a_{12} & a_{11} 
\end{bmatrix}
\]

What is the correct approach for this?

Answer 1

The notation \[ and \] puts TeX in (display) math mode, meaning you don't have to enter math mode again using $ ... $. In fact, this causes an error. Barring that, the alignment should be fine:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
  M^{-T} = \frac{1}{|\det(M)|}
    \begin{bmatrix}
      \phantom{-}a_{22} & -a_{21} \\
      -a_{12} & \phantom{-}a_{11} 
    \end{bmatrix}
\]
\end{document}​

In the above example I've played around with some parameters:

• Right-aligned all the entries in the bmatrix (using \phantom) so positive and negative scalar quantities line up. This may be personal preference.
• amsmath provides \text which sets its argument in the correct font size (although it doesn't matter in your specific case) in text mode. You'll notice a difference in the spacing between det and \text{det} in math mode. Moreover, in math mode, det is intepreted as d multiplied by e, multiplied by t, which is not correct. LaTeX also provides \det natively for this, defined as a math operator (and therefore has a marginally different spacing, but better in general).