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I'd like to write the matrix equation A = (x y) B (x y)^T, where (x y)^T is written as a column vector and B is a 2x2 matrix written as such. I can (almost) do this by, for example, the following:

A = (x \ y) \begin{pmatrix} a & b \\ c & d \end{pmatrix} \begin{pmatrix} x \\ y \end{pmatrix}

But I'd like the row vector (x y) to be aligned with the first row of the matrix B. How do I do this?

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up vote 5 down vote accepted

For consistency's sake, I would also use a pmatrix for the first matrix; then you can wrap the first pmatrix inside a matrix with an empty second row:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
A = 
  \begin{matrix}\begin{pmatrix}x & y\end{pmatrix}\\\mbox{}\end{matrix}
  \begin{pmatrix} a & b \\ c & d \end{pmatrix} 
  \begin{pmatrix} x \\ y \end{pmatrix}
\]

\end{document}

enter image description here

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Come to think of it, is this type of equation typically formatted to have the row vector in line with the first row of the matrix? –  Matt Jan 6 '12 at 15:46
1  
@Matt: No. I have never seen it like that at least. Row vectors are usually just set in the same way as scalars. –  Roelof Spijker Jan 6 '12 at 15:55
    
Funny, I always do it this way by hand, but never considered typesetting it like this. –  qubyte Jan 6 '12 at 16:06
    
@Matt This would be mistaken for a kind of exponent; leave it in the middle. What would you do for the product of a 2x4 matrix by a 4x3 one? –  egreg Jan 6 '12 at 16:12
    
@MarkS.Everitt, I always do it that way by hand too, but it just doesn't look right when typesetting. Sorry! –  Matt Jan 6 '12 at 16:13
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