Connecting parts of equations with lines

Question

When for example collecting terms in writing out an expression, it is often useful to indicate what terms are similar by drawing lines between them. Take for example

\documentclass{article}
\begin{document}
\[(a+b)(a+b)-(a+b)(a-b)=aa+ab+ba+bb-(aa-ab+ba-bb)=2ab+2bb.\]
\end{document}

This would be a lot easier to read with the following lines:

Is there a way of doing this in LaTeX (the only thing that comes to mind is editing the shape of \underbrace and \overbrace, but that doesn't seem best practice)?

score 4 · Accepted Answer · answered Sep 04 '15 at 16:25

You can use the tikzmark library from TikZ (compile twice so the lines reach their final position):

The code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\newcommand\JoinUp[4][10pt]{
\draw
  ([shift={#2}]pic cs:start#4) -- ++(0pt,#1) -| ([shift={(#3)}]pic cs:end#4);
}
\newcommand\JoinDown[4][10pt]{
\draw
  ([shift={#2}]pic cs:start#4) -- ++(0pt,-#1) -| ([shift={(#3)}]pic cs:end#4);
}

\begin{document}

\[
(a+b)(a+b)-(a+b)(a-b)=
\tikzmark{starta}aa+\tikzmark{startc}ab+\tikzmark{endc}ba+
\tikzmark{startd}bb-(\tikzmark{enda}aa-\tikzmark{startb}ab+
\tikzmark{endb}ba-\tikzmark{endd}bb)=2ab+2bb.
\]

\begin{tikzpicture}[remember picture,overlay,line width=1.5pt]
\JoinUp{(6pt,10pt)}{(6pt,10pt)}{a}
\JoinUp{(5pt,10pt)}{(5pt,10pt)}{b}
\JoinDown{(5pt,-2pt)}{(5pt,-2pt)}{c}
\JoinDown{(5pt,-2pt)}{(5pt,-2pt)}{d}
\end{tikzpicture}

\end{document}

score 1 · Answer 2 · edited Apr 13 '17 at 12:35

A small addition to Gonzalo's answer.

Just to cut down on the typing and making the whole thing scale with the font size of the document, I figured it would be nice to incorporate the answer to Get current font size as length and make every size scale accordingly:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{tikzmark}

\newlength{\mytextsize}
\makeatletter
      \setlength{\mytextsize}{\f@size pt}
\makeatother

\newcommand{\JoinUp}[5]{\begin{tikzpicture}[remember picture,overlay,line width=0.05\mytextsize]
    \draw([shift={(#1\mytextsize,#2\mytextsize)}]pic cs:start#5) -- ++(0pt,0.7\mytextsize) -| ([shift={(#3\mytextsize,#4\mytextsize)}]pic cs:end#5);
    \end{tikzpicture}}
\newcommand{\JoinDown}[5]{\begin{tikzpicture}[remember picture,overlay,line width=0.05\mytextsize]
    \draw([shift={(#1\mytextsize,#2\mytextsize)}]pic cs:start#5) -- ++(0pt,-0.7\mytextsize) -| ([shift={(#3\mytextsize,#4\mytextsize)}]pic cs:end#5);
    \end{tikzpicture}}

\begin{document}
\[(a+b)(a+b)-(a+b)(a-b)=\tikzmark{starta}aa+\tikzmark{startc}ab+\tikzmark{endc}ba+\tikzmark{startd}bb-(\tikzmark{enda}aa-\tikzmark{startb}ab+\tikzmark{endb}ba-\tikzmark{endd}bb)=2ab+2bb.\]
\JoinUp{0.5}{1}{0.5}{1}{a}
\JoinUp{0.5}{1}{0.5}{1}{b}
\JoinDown{0.5}{-0.4}{0.5}{-0.4}{c}
\JoinDown{0.5}{-0.4}{0.5}{-0.4}{d}
\end{document}

This changes the \JoinUp and \JoinDown commands a little, as the displacements are now given in terms of the font size.

Note: this does not scale with the local font size, as the \mytextsize length is defined in the preamble.

Connecting parts of equations with lines

2 Answers2