Argthtjtr

I want to make a comment box like the image below that adjusts in size as I write up different examples. My MWE is not much

documentclass{article}

usepackage{amsmath, mathtools,tikz}



begin{document}



begin{align*}              

displaystyleint 3x^{2}(x^{3}+1)^{10} dx &=int (textcolor{red} 

   {x^{3}+1})^{10} textcolor{blue}{3x^{2} dx} \

                                       &=int textcolor{red}{u}^{10} 

   textcolor{blue}{du}

end{align*}



end{document}

This outputs:

I am trying to recreate with a black box drawn around the orange shaded area.

Using tikzmark, it is not so difficult:

documentclass{article}

usepackage{amsmath, mathtools,tikz}

usetikzlibrary{tikzmark,calc}

begin{document}



begin{align*}              

displaystyleint 3x^{2}(x^{3}+1)^{10} dx &=int (textcolor{red} {x^{3}+1})^{10} textcolor{blue}{3x^{2} dx} tikzmark{eqt}\

&=int textcolor{red}{u}^{10}textcolor{blue}{du}

end{align*}



begin{tikzpicture}[overlay, remember picture]

coordinate (x) at ($(pic cs:eqt)+(1,0)$);

draw (x)--($(x)+(3,0)$);

draw[red] (x) node[above right] {Substitution};

draw ($(x)+(3,0)$) node[right,draw,fill=orange!50] {% 

% Remove `draw' option if you don't want the box

$begin{aligned}

    u&=x^3+1,\

    du&=3x^2dx

end{aligned}$};

end{tikzpicture}



end{document}

This very nice and creative solution is by @marmot:

documentclass[fleqn]{article}

usepackage{amsmath,tikz}

usetikzlibrary{tikzmark}

begin{document}



begin{align*}              

displaystyleint 3x^{2}(x^{3}+1)^{10},mathrm{d}x 

&tikzmarknode{eq1}{=}int (textcolor{red} {x^{3}+1})^{10} 

textcolor{blue}{3x^{2}, mathrm{d}x} \

&tikzmarknode{eq2}{=}int textcolor{red}{u}^{10},textcolor{blue}{mathrm{d}u}

end{align*}



begin{tikzpicture}[overlay, remember picture]

path (eq1) -- (eq2) coordinate[midway] (aux);

draw[latex-] (eq2) -- (aux) -- ++ (5,0) node[above left] {substitute}

node[right,fill=orange!30] {%

$begin{aligned}

    u&=x^3+1\

    mathrm{d}u&=3x^2,mathrm{d}x

end{aligned}$};

end{tikzpicture}

end{document}

Without tikz, I define an extensible xmathline, centred on the math axis, which accepts writing above and below it, like xrightarrow, and add a simple colorbox:

documentclass[svgnames]{article}

usepackage{amsmath, mathtools,tikz}

usepackage{xcolor}

usepackage{bigstrut}



    newcommand{xmathline}[2]{%

      ext@arrow 0099xmathlinefill@{#1}{#2}}%

    newcommand{xmathlinefill@}{%

      arrowfill@{relbarbigstrut}relbarrelbar}

    makeatother



begin{document}



begin{align*}

displaystyleint 3x^{2}(x^{3}+1)^{10} dx &=int (textcolor{red}

   {x^{3}+1})^{10} textcolor{blue}{3x^{2} dx}

   quadxmathline{text{color{Crimson}substitutionquad}}colorbox{NavajoWhite! 60}{$begin{aligned} u & = x^3 + 1 \ du & =3x^2,dx end{aligned} $} \%

                                       &=int textcolor{red}{u}^{10}

   textcolor{blue}{du}

end{align*}



end{document}