# Drawing an angled arrow pointing to text inside equation

I am trying to insert arrows between the terms of an equation explaining the partial results achieved. For example, in the following equation (I manually drew the arrows), V_i \frac{g_m}{1+g_mR_s} would give I_d, while V_i \frac{g_m}{1+g_mR_s} R_d would be equal to V_{R_d} and so on:

Could anyone help me come up with a solution?

A nested stack. I introduce \dnAr[<size>]{<label>} to do so, where <size> is stuff like \bigg (the default). In the 2nd example, I exercise the optional argument to \dnAr.

\documentclass{report}
\usepackage{stackengine,graphicx}
\newcommand\dnAr[2][\bigg]{\ensurestackMath{%
\stackengine{-0.3pt}{#1\vert}{%
\stackengine{1pt}{\rotatebox{55}{$\leftarrow\mkern-2mu$}}{%
\mkern-5mu\scriptscriptstyle#2}{U}{l}{F}{T}{S}}{U}{r}{F}{T}{S}%
}}
\begin{document}
$V_u = V_i \frac{g_m}{1 + g_m R_s} \dnAr{I_d} R_d \dnAr{V_{R_d}}(-1)\dnAr{V_u}$
$V_u = V_i \frac{g_m}{1 + g_m R_s} \dnAr{I_d} R_d \dnAr[\big]{V_{R_d}}(-1)\dnAr[]{V_u}$
\end{document}

Here's a version with 45 degree angled arrow and a tighter under-label:

\documentclass{report}
\usepackage{stackengine,graphicx}
\newcommand\dnAr[2][\bigg]{\ensurestackMath{%
\stackengine{-0.7pt}{#1\vert}{%
\stackengine{-1pt}{\rotatebox{45}{$\leftarrow\mkern-1.5mu$}}{%
\mkern-5mu\scriptscriptstyle#2}{U}{l}{F}{T}{S}}{U}{r}{F}{T}{S}%
}}
\begin{document}
$V_u = V_i \frac{g_m}{1 + g_m R_s} \dnAr{I_d} R_d \dnAr{V_{R_d}}(-1)\dnAr{V_u}$
$V_u = V_i \frac{g_m}{1 + g_m R_s} \dnAr{I_d} R_d \dnAr[\big]{V_{R_d}}(-1)\dnAr[]{V_u}$
\end{document}

• @RenatoArino You are most welcome. I have added a variant that may be more to your taste. – Steven B. Segletes Apr 28 '16 at 18:57

A Tikz version.

## Code

\documentclass{article}
\usepackage{tikz}

\newcommand\arrd[1]{
\tikz[baseline, inner xsep=-1cm]{
\draw[->] (0,.4) -- (0,-.2) -- (-.1,-.3) node[below, font=\tiny] {#1};
}\ % for spacing
}

\begin{document}

$V_u = V_i \frac{g_m}{1+g_mR_s} \arrd{$id$} R_d \arrd{$V_{R_d}$} (-1) \arrd{$V_u$}$

\end{document}