Cfrtjryk

I want to use a vertical bar to comment on solving a simple equation.

Do you know a way to do that with LaTeX ?

Yes, it's possible:

documentclass{article}

usepackage{amsmath,array}



begin{document}



begin{equation*}

renewcommand{arraystretch}{1.2}

setlength{arraycolsep}{0pt}

begin{array}{

  >{displaystyle}r   % right aligned

  >{displaystyle{}}l % left aligned

  @{hspace{4em}}     % spacing

  |                   % vertical rule

  @{hspace{2em}}     % spacing

  >{displaystyle}r   % comments

}

2x+2 &= 7x+3 & -2 \

2x   &= 7x+1 & -7x \

-5x  &= 1    & div(-5) \

x    &= boxed{-frac{1}{5}} & \

end{array}

end{equation*}



end{document}

If you're satisfied by the spacing, you can make this into an environment:

documentclass{article}

usepackage{amsmath,array}



newenvironment{solve}

 {%

  renewcommand{arraystretch}{1.2}%

  setlength{arraycolsep}{0pt}%

  begin{array}{

    >{displaystyle}r   % right aligned

    >{displaystyle{}}l % left aligned

    @{hspace{4em}}     % spacing

    |                   % vertical rule

    @{hspace{2em}}     % spacing

    >{displaystyle}r   % comments

  }%

 }%

 {end{array}}



begin{document}



begin{equation*}

begin{solve}

2x+2 &= 7x+3 & -2 \

2x   &= 7x+1 & -7x \

-5x  &= 1    & div(-5) \

x    &= boxed{-frac{1}{5}} & \

end{solve}

end{equation*}



end{document}

It almost seems better to use a different format, which allows you easier control of numbering elements within a sequence if needed.

documentclass{article}



usepackage{amsmath}



newcommand{eqcomment}[1]{& triangleright~ & #1}



begin{document}



begin{align*}

  2x + 2 &= 7x + 3 eqcomment{-2}       \

      2x &= 7x + 1 eqcomment{-7x}      \

     -5x &= 1      eqcomment{div(-5)} \

       x &= boxed{-frac{1}{5}}

end{align*}



end{document}

One can play around with the alignment and notation.

Two other possibilities:

documentclass{article}

usepackage{showframe}

renewcommand{ShowFrameLinethickness}{0.3pt}

usepackage{eqparbox}

usepackage{amsmath,array}



begin{document}



begin{flalign*}

qquad & & 2x+2 &= 7x+3 &smash{rule[-4.5baselineskip]{0.5pt}{5.5baselineskip}} quadeqmakebox[B][r]{$ -2 $} & \

 & & 2x &= 7x+1 &eqmakebox[B][r]{$ -7x $}& \

 & & -5x &= 1 & eqmakebox[B][r]{$ div(-5) $} & \

 & & x &= boxed{-frac{1}{5}} & &

end{flalign*}



begin{alignat*}{2}

 qquad2x+2 &= 7x+3 &hspace{6em}smash{rule[-5baselineskip]{0.5pt}{6baselineskip}} qquad-2 & \

 2x &= 7x+1 & -7 x & \

 -5x &= 1 &div(-5) & \

 x &= boxed{-frac{1}{5}} & &

end{alignat*}



end{document}