[Tex/LaTex] How to box two lines inside align*

alignboxesmath-mode

I asked a similar question today except it was targeted at one particular line inside the align* environment. Now I want to enclose two lines. The failed attempt below makes my intention clear:

\begin{align*}
\ce{K_a $\times$ K_b} &= \ce{K_w}\\
-\log(\ce{K_a $\times$ K_b}) &= -\log\ce{K_w}\\
 -( \log(\ce{K_a}) + \log(\ce{K_b}) ) &= -\log\ce{K_w} & \text{(Using log law for LHS.)} \\
 - \log(\ce{K_a}) - \log(\ce{K_b}) &= -\log\ce{K_w}\\
\fbox{
 \begin{minipage}{\textwidth}
\ce{pK_a} + \ce{pK_b} &= \ce{pK_w}\\
&=14.00 \text{ (at \SI{25}{\celsius})}\\
\end{minpage}
}
\end{align*}

\Aboxed from mathtools pkg was a great solution, you didn't have to specify mandatory parameters and it was as simple as Aboxed{ } – alas it only works for single line boxes.

As for \Aboxed I don't want to be asked how wide the box should be – it should be automatically determined.

As usual, I prefer predefined solutions, and less obstrusive code to achieve the desired effect. The less clutter the better.

P.S. By the way, this manual seems to suggest \mbox, and anologously \fbox ought to draw frames around the enclosed content. Previously I could not get \mbox to display visible frames…

Best Answer

Here an inspiration with tikz

(everyone is welcome to modify the example -- i am not familiar with tikz)

\documentclass{article}
\usepackage{mathtools}
\usepackage{mhchem}
\usepackage{siunitx}

\usepackage{tikz}
\newcounter{markeq}
\setcounter{markeq}{0}
\newcommand*\bmarkeq{%
   \stepcounter{markeq}%
   \tikz[remember picture]\node(startframe-\themarkeq){\strut};}
\newcommand*\emarkeq{%
   \begin{tikzpicture}[remember picture,overlay]
     \node (endframe-\themarkeq){\strut};
     \draw[,red,opacity=0.8] (startframe-\themarkeq.north) rectangle (endframe-\themarkeq.south);
   \end{tikzpicture}%
}

\begin{document}


\begin{align*}
\ce{K_a $\times$ K_b} &= \ce{K_w}\\
-\log(\ce{K_a $\times$ K_b}) &= -\log\ce{K_w}\\
 -( \log(\ce{K_a}) + \log(\ce{K_b}) ) &= -\log\ce{K_w} & \text{(Using log law for LHS.)} \\
 - \log(\ce{K_a}) - \log(\ce{K_b}) &= -\log\ce{K_w}\\
\bmarkeq\ce{pK_a} + \ce{pK_b} &= \ce{pK_w}\\
&=14.00 \text{ (at \SI{25}{\celsius})}\emarkeq
\end{align*}


\end{document}

EDIT

With the very great idea of egreg you get:

\documentclass{article}
\usepackage{mathtools}
\usepackage{mhchem}
\usepackage{siunitx}

\usepackage{tikz}
\newcounter{markeq}
\setcounter{markeq}{0}

\newcommand{\pstrut}[1]{\vrule height0pt depth0pt width0pt #1 \fboxsep}
\newcommand*\bmarkeq{\stepcounter{markeq}%
  \tikz[remember picture]\node(startframe-\themarkeq){\pstrut{height}};%
  \kern\fboxsep}
\newcommand*\emarkeq{\kern\fboxsep
  \begin{tikzpicture}[remember picture,overlay]
    \node (endframe-\themarkeq){\pstrut{depth}};
    \draw[,red,opacity=0.8] (startframe-\themarkeq.north) 
      rectangle (endframe-\themarkeq.south);
  \end{tikzpicture}%
}


\begin{document}


\begin{align*}
\ce{K_a $\times$ K_b} &= \ce{K_w}\\
-\log(\ce{K_a $\times$ K_b}) &= -\log\ce{K_w}\\
 -( \log(\ce{K_a}) + \log(\ce{K_b}) ) &= -\log\ce{K_w} & \text{(Using log law for LHS.)} \\
 - \log(\ce{K_a}) - \log(\ce{K_b}) &= -\log\ce{K_w}\\
\bmarkeq\ce{pK_a} + \ce{pK_b} &= \ce{pK_w}\\
&=14.00 \text{ (at \SI{25}{\celsius})}\emarkeq
\end{align*}


\end{document}

enter image description here

Related Solutions

LaTeX – Why \mbox Does Not Draw a Box Inside align*

Perhaps you are trying to place a box around the last equation. One way to do that is to use Aboxed from the mathtools package:

\documentclass{article}
\usepackage{mathtools}
\usepackage{mhchem}
\begin{document}
    \begin{align*} 
    \ce{K_w} &= \ce{[H3O+][OH^-]} \\
    -\log \ce{K_w} &= -\log ( \ce{[H3O+][OH^-]} ) \\
    -\log \ce{K_w} &= - ( \log \ce{[H3O+]} + \log \ce{[OH^-]} )  & \text{(Using log law for RHS.)}\\
    -\log \ce{K_w} &= -  \log \ce{[H3O+]} - \log \ce{[OH^-]} \\
    -\log \ce{K_w} &= -  \log \ce{[H3O+]} + - \log \ce{[OH^-]} \\
    \Aboxed{ \ce{pK_w} &= \ce{pH} + \ce{pOH} }
    \end{align*}
\end{document}

enter image description here

While it is true that the descriptions of mbox tell you that it creates a box just wide enough to hold the text, this is not a box that gets drawn. It is a virtual "box" in the TeX sense that does not get split across lines.

[Tex/LaTex] Box or [ around multiple lines inside align

An option using a \parbox inside a \fcolorbox inside a \makebox, and the whole construct inside \intertext; some changes to the original mark-up were done (in particular, the code was greatly simplified), but the original alignment of expressions before and after the box was preserved:

\documentclass{article}
\usepackage{mathtools}
\usepackage{tikzpagenodes}
\usetikzlibrary{tikzmark}

\begin{document}

Let:
\begin{align*}
G_{\delta_r}^{\beta} &= \frac{n_1 s + n_0}{s^2 + d_1 s + d_0} = \frac{0.01956 s + 7.01}{s^2 + 0.802 s + 6.489} \\
\text{step} \left(G_{\delta_r}^{\beta}(s) \right) = \frac{G_{\delta_r}^{\beta}(s)}{s} &= \frac{1}{s} \cdot \frac{n_1 s + n_0}{s^2 + d_1 s + d_0}
\intertext{%
\makebox[\linewidth][c]{\fcolorbox{blue}{white}{\parbox{\linewidth}{%
Expanding with partial fractions:
\[
\frac{x_1}{s} + \frac{x_{21}s + x_{20}}{s^2 + d_1 s + d_0} = 
\frac{1}{s} \cdot \frac{n_1 s + n_0}{s^2 + d_1 s + d_0}
\]
Removing denominator:
\[
x_1(s^2 + d_1 s + d_0) + s(x_{21} s + x_{20}) = n_1 s + n_0 
\]
Has solutions:
\[
x_{21} = \frac{-n_0}{d_0} = -1.08\qquad
x_1 = \frac{n_0}{d_0} = 1.08\qquad
x_{20} = n_1 - \frac{n_0 d_1}{d_0} = -0.847
\]
Hence:}}}
}
\frac{G_{\delta_r}^{\beta}(s)}{s} &= \frac{1.08}{s} - \frac{1.08s + 0.847}{s^2 + 0.8-2s + 6.489} \\
\shortintertext{Completing the square:}
\frac{G_{\delta_r}^{\beta}(s)}{s} &= \frac{1.08}{s} - \frac{1.08s + 0.847}{(s + 0.401)^2 + 6.328} \\
\shortintertext{Inverse Laplace:}
\mathcal{L}^{-1}\left\{ \frac{G_{\delta_r}^{\beta}(s)}{s} \right\} &=  1.08 - e^{-.401t} \left(1.08 \cos\alpha + \frac{0.847}{\sqrt{6.328}} \sin\alpha \right), 
\end{align*}
where $\alpha=t\sqrt{6.328}$.

\end{document}

enter image description here

Another option would be to use the tikzmark library; the idea is to use \tikzmark to place marks at the appropriate locations and then to use those marks to draw the blue rectangle. Below I show this approach and also illustrate another option for the mark-up (of course, this approach will also work with the above approach or with any other variation):

\documentclass{article}
\usepackage{mathtools}
\usepackage{tikzpagenodes}
\usetikzlibrary{tikzmark}

\begin{document}

Let:
\begin{align*}
G_{\delta_r}^{\beta} &= \frac{n_1 s + n_0}{s^2 + d_1 s + d_0} = \frac{0.01956 s + 7.01}{s^2 + 0.802 s + 6.489} \\
\text{step} \left(G_{\delta_r}^{\beta}(s) \right) = \frac{G_{\delta_r}^{\beta}(s)}{s} &= \frac{1}{s} \cdot \frac{n_1 s + n_0}{s^2 + d_1 s + d_0}
\end{align*}         
\tikzmark{start}Expanding with partial fractions:
\[
\frac{x_1}{s} + \frac{x_{21}s + x_{20}}{s^2 + d_1 s + d_0} = 
\frac{1}{s} \cdot \frac{n_1 s + n_0}{s^2 + d_1 s + d_0}
\]
Removing denominator:
\[
x_1(s^2 + d_1 s + d_0) + s(x_{21} s + x_{20}) = n_1 s + n_0 
\]
Has solutions:
\[
x_{21} = \frac{-n_0}{d_0} = -1.08\qquad
x_1 = \frac{n_0}{d_0} = 1.08\qquad
x_{20} = n_1 - \frac{n_0 d_1}{d_0} = -0.847\tikzmark{end}
\]
Hence:
\begin{align*} 
\frac{G_{\delta_r}^{\beta}(s)}{s} &= \frac{1.08}{s} - \frac{1.08s + 0.847}{s^2 + 0.8-2s + 6.489} \\
\shortintertext{Completing the square:}
\frac{G_{\delta_r}^{\beta}(s)}{s} &= \frac{1.08}{s} - \frac{1.08s + 0.847}{(s + 0.401)^2 + 6.328} \\
\shortintertext{Inverse Laplace:}
\mathcal{L}^{-1}\left\{ \frac{G_{\delta_r}^{\beta}(s)}{s} \right\} &=  1.08 - e^{-.401t} \left(1.08 \cos(t\sqrt{6.328}) + \frac{0.847}{\sqrt{6.328}} \sin(t\sqrt{6.328}) \right) 
\end{align*}

\begin{tikzpicture}[remember picture,overlay]
\draw[line width=1.5pt,blue]
  ([xshift=-1ex,yshift=2.5ex]current page text area.west|-{pic cs:start})
  rectangle
  ([yshift=-2.5ex]current page text area.east|-{pic cs:end});
\end{tikzpicture}

\end{document}

enter image description here

Best Answer

Related Solutions

LaTeX – Why \mbox Does Not Draw a Box Inside align*

[Tex/LaTex] Box or [ around multiple lines inside align

Related Question