[Tex/LaTex] Using tables to typeset proof – how to align equal signs

aligntables

I'm a student teaching myself to use LaTeX (and related tools) to type up my homework. It's going great, and I'm already very happy with what I've been able to do so far. Though, I have questions.

I'm trying to type up a proof with steps in one column and explanations in a second column, separated by a nice vertical bar. A two column table has gotten me most of the way there:

\documentclass[a4paper,12pt]{article}
\usepackage{amsthm}
\begin{document}
We want to show that -2 = 2.
\begin{proof}
\begin{tabular}{r|l}
$-2 = 2$ & assuming the conclusion \\
$(-2)^2 = 2^2$ & square both sides \\
$4$ = $4$ & as desired
\end{tabular}

\end{proof}
\end{document}

However, the equals signs in the left column don't line up. Ideally, I'd like to be able to line them up as I would in an align environment or similar. Can this be done?

Moreover, I don't really like having to repeat the $ on every row of the left column. Is there a way to automate that?

Thanks in advance for any answers, this is my first question here too, so please let me know if I've accidentally violated any mores of this community.

Best Answer

Do it simpler.

\documentclass[12pt,a4paper]{article}
\usepackage{mathtools}   % loads »amsmath«
\usepackage{amsthm}

\begin{document}
  We want to show that $-2=2$.
  \begin{proof}
    \begin{align*}
      -2 &= 2 && \smash{\Big|}\text{ assuming the conclusion} \\
      (-2)^2 &= 2^2 && \smash{\Big|}\text{ square both sides} \\
      4 &= 4 && \smash{\Big|}\text{ as desired}
    \end{align*}
  \end{proof}
\end{document}

Discussion

The documentation amsmath.pdf is not so clear about this structure saying

The split structure should constitute the entire body of the enclosing structure, apart from commands like \label that produce no visible material.

in the section "Split equations with alignment", in contrast to the description under gather

Any equation in a gather may consist of a \begin{split} ... \end{split} structure—...

However, the accompanying filed testmath.pdf contains two explicit examples of multiple splits within an align. What you can't do is enclose such a split construction inside e.g. a \left ... \right construction.

Note that there is the package breqn which contains code that allows complicated alignment between various blocks, but it alters fundamental constructions in math mode, and so has a number of compatibility issues.

[Tex/LaTex] Align all equal signs in the document

Starting with my answer on align separate equations, I found I needed to change a few things, specifically because the OP for this question wanted to include multiline equations. As such, I had to reinstate \arraystretch to a non-zero value (in fact, I set it to 1.2). But with that, I had to do some \vspace manipulations going into and out of the algneqn environment I created.

But in the end, all the equal signs are aligned, per the OP's request. Setting the values of \leftalgn and \rightalgn will determine where on the overall line the alignment tab will be set.

\documentclass{article}
\usepackage{amsmath}
\def\leftalgn{0.3}\def\rightalgn{0.6}
\def\algnrow{\rule{\leftalgn\textwidth}{0ex}&\rule{\rightalgn\textwidth}{0ex}}
% CONSTRAINTS:
% equation label must fit in {1 -\leftalgn -\rightalgn}\textwidth
% \leftalgn must be larger than any text to left of align character
% \rightalgn must be larger than any text to right of align character
\newenvironment{algneqn}{%
  \vspace{-1.6\baselineskip}%
  \arraycolsep=0ex\renewcommand\arraystretch{1.2}%
  \begin{equation}%
  \begin{array}{rl}%
  \algnrow\\%
}%
 {\\\algnrow%
  \end{array}\vspace{-\baselineskip}%
  \end{equation}\ignorespacesafterend%
}
\def\snug#1{\vspace*{-#1\baselineskip}}
\begin{document}
\begin{algneqn}    
F_L^{\rightarrow}   & =     q ( E_{eFeld}^{\rightarrow} + v^{\rightarrow} \times B^{\rightarrow} )
    \end{algneqn}

much text

    \begin{algneqn}
    F_{Lx}  & =     q ( E_{eFeld} + v_{gx} * B )
    \end{algneqn}

    little text

    \begin{algneqn}
    T   & =     F_{L1} - F_{L2}                                         \\
        & =     q ( E_{eFeld} + v_{g1} * B - E_{eFeld} - v_{g2} * B )   \\
        & =     q*B ( v_{g1} - v_{g2} )         
    \end{algneqn}                     

    \subsection{new subsection}

    very little text

    \begin{algneqn}
    P_{Photon}  & =     hf * nAc                \\
                & =     \dfrac{E_{Photon}}{t}    
    \end{algneqn}

    more text

    \begin{algneqn}
    F_{0}       & =     2nhfA               \\
                & =     \dfrac{2 P_{Photon}}{c}  
    \end{algneqn}
\end{document}

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Best Answer

Related Solutions

Align Equal Signs – How to Align Two Split Environments at the Equals Sign in LaTeX

Discussion

[Tex/LaTex] Align all equal signs in the document

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