alignlengths
If I want to know the width of a text I use
\settowidth{\MyLength}{bla}.
Is there something similar for equations?
More specifically I have
\begin{align}
a & b & c & d\\
c & d & c & d
\end{align}
and I would like to have the width of this align section.
How can I get this?
In many situations, a comment can be added in the following way:
\begin{align*}
A &= B && \text{(because pigs fly)} \\
&= C && \text{(since donkeys are not pigs)} \\
&= D && \text{(because I say so)}
\end{align*}
so that the comments are (left) aligned to each other. However, your case is different: the comment would be only in the last two lines and the fifth one is quite long. The fifth line makes it hard to set the comments so they stick out as such.
Probably the most amenable solution is to defer the comments to the text below the alignment. These are tough decisions, anyway. What about this?
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\makebox[2.8em][l]{$\displaystyle P(X_l=x_l,\ldots ,X_{l+k}=x_{l+k})$} \\
&= \int_0^1 f(x_l)\cdots f(x_{l+k})\,dp\\
&= \int_0^1 \prod_{i=1}^k p^{1\{x_i=1\}}(1-p)^{1\{x_i=0\}}\,dp\\
&= \int_0^1 p^t(1-p)^{k-t}dp\\
&= \int_0^1 p^{t+1-1}(1-p)^{k-t+1-1}\,dp\\
&= \dfrac{\Gamma(k-t+1)\Gamma(t+1)}{\Gamma(k+2)}\int_0^1\dfrac{\Gamma(k+2)}{\Gamma(k-t+1)\Gamma(t+1)} p^{t+1-1}(1-p)^{k-t+1-1}\,dp\\
&= \dfrac{\Gamma(k-t+1)\Gamma(t+1)}{\Gamma(k+2)} \\
&= \dfrac{t!(k-t)!}{(k+1)!}
\end{align*}
Notice that the last integral is $\Gamma(t+1,k-t+1)$ and $k$ is an integer.
\end{document}
amsmath provides \intertext{} for this kind of purpose. For example,
\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\begin{frame}
\frametitle {MWE: Align two set of equations}
\begin{align*}
\sigma^2&=(a+b)^2-(a+c)^2 \\
&=(a^2+b^2-2ab)-(a^2+c^2-2ac)\\
\intertext{Cancel out common terms, then}
\sigma^2&= b^2-c^2+2(ac-ab)
\end{align*}
\end{frame}
\end{document}
Best Answer
One can get the sum of the widths of the "equation" parts, as the spacing between alignment groups is computed dynamically and is stored nowhere.
The column widths are stored in a special format in the macro
\maxcolumn@widths, which is available at the end of the construction, when the conditional\ifmeasuring@is false. The format iswhere each
<dimen>is the maximum width of the respective column (alignbuildsrlrl...alignments).One may add to this the computation of the horizontal space really used from the left to the right.