Underlining an equation in an align block

Question

I'm trying to set up a system of equations and want to underline the last equation, but can't figure it out. Here's what I have:

\begin{alignat*}{4}
&F_n &- mg &- F_a\sin 32^\circ &= 0\\
-&\mu F_n & &+ F_a\cos 32^\circ &= 0\\
\end{alignat*}

After this I want line, and then the answer. I tried \underline, but that gave me errors. I also tried using \line, but that would give errors in the block or be too far away from the equations outside of the block.

Any help would be greatly appreciated!

Answer 1

\documentclass{article}
\usepackage{amsmath}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{c}
\parbox{4.3cm}{{\begin{alignat*}{4}
&F_n &{}- mg &- F_a\sin 32^\circ &= 0\\
-&\mu F_n & &+ F_a\cos 32^\circ &= 0\\[-25pt]
\end{alignat*}}}\\\bottomrule
\end{tabular}
\end{document}

Answer 2

This is a job for the \tikzmark macro if I interpreted correctly the request. The beauty of such an approach is that the rule underlying the equation fit exactly the dimensions of the equation.

Code:

\documentclass{article}
\usepackage{tikz,amsmath}


% to change colors
\newcommand{\bordercol}{red}

%% code by Andrew Stacey 
% http://tex.stackexchange.com/questions/51582/background-coloring-with-overlay-specification-in-algorithm2e-beamer-package#51582

\makeatletter
\tikzset{%
     remember picture with id/.style={%
       remember picture,
       overlay,
       save picture id=#1,
     },
     save picture id/.code={%
       \edef\pgf@temp{#1}%
       \immediate\write\pgfutil@auxout{%
         \noexpand\savepointas{\pgf@temp}{\pgfpictureid}}%
     },
     if picture id/.code args={#1#2#3}{%
       \@ifundefined{save@pt@#1}{%
         \pgfkeysalso{#3}%
       }{
         \pgfkeysalso{#2}%
       }
     }
   }

   \def\savepointas#1#2{%
  \expandafter\gdef\csname save@pt@#1\endcsname{#2}%
}

\def\tmk@labeldef#1,#2\@nil{%
  \def\tmk@label{#1}%
  \def\tmk@def{#2}%
}

\tikzdeclarecoordinatesystem{pic}{%
  \pgfutil@in@,{#1}%
  \ifpgfutil@in@%
    \tmk@labeldef#1\@nil
  \else
    \tmk@labeldef#1,(0pt,0pt)\@nil
  \fi
  \@ifundefined{save@pt@\tmk@label}{%
    \tikz@scan@one@point\pgfutil@firstofone\tmk@def
  }{%
  \pgfsys@getposition{\csname save@pt@\tmk@label\endcsname}\save@orig@pic%
  \pgfsys@getposition{\pgfpictureid}\save@this@pic%
  \pgf@process{\pgfpointorigin\save@this@pic}%
  \pgf@xa=\pgf@x
  \pgf@ya=\pgf@y
  \pgf@process{\pgfpointorigin\save@orig@pic}%
  \advance\pgf@x by -\pgf@xa
  \advance\pgf@y by -\pgf@ya
  }%
}
\makeatother

\newcommand{\tikzmarkin}[2][]{%
      \tikz[remember picture,overlay,baseline=1ex]
      \draw[line width=1pt,#1]
      (pic cs:#2) -- (0,0)
      ;}

\newcommand\tikzmarkend[2][]{%
\tikz[remember picture with id=#2,baseline=1ex] #1;}

\begin{document}
\begin{alignat*}{4}
&F_n &- mg &- F_a\sin 32^\circ &= 0\\
\tikzmarkin[red]{a}-&\mu F_n & &+ F_a\cos 32^\circ &= 0\tikzmarkend{a}\\
& F_n &- mg &- F_a\sin 32^\circ &= 0\\
\end{alignat*}
Here is my answer. Last equation is not needed, but it is a test to see where it will be placed with respect to the underlying line.

\begin{alignat*}{4}
&F_n &- mg &- F_a\sin 32^\circ &= 0+a+b+c\\
\tikzmarkin[blue, line width=2pt]{b}-&\mu F_n & &+ F_a\cos 32^\circ &= 0+a+b+c\tikzmarkend{b}\\
\end{alignat*}

\end{document}

Result:

It is based on the improved version by Andrew Stacey with a bit modification to just draw a line in which it's possible to customize the aspect.

Usage: \tikzmarkin{identifier} in the starting point, \tikzmarkend{identifier} in the ending point in which you have to draw the line. Optional argument for aspect just in \tikzmarkin: \tikzmarkin[blue, line width=2pt]{b}.

Notice that you should compile twice the document to get the right result (first run to set marker positions, second one to effectively draw the line). Last remark: the mark identifiers should be univoque in the document.