Illustrate distributive law with “broken” arrows

Question

I would like to obtain the following formatting.

Answer 1

You could (ab-) use chemmacros's redox macros:

% arara: pdflatex
% arara: pdflatex
\documentclass{scrartcl}
\usepackage{chemmacros}

\begin{document}

\begin{equation}
 \OX{first,$k$}(\OX{second,$a$}+\OX{third,$b$}) = ka + kb
\end{equation}
\redox(first,second)[->]{$\times$}
\redox(first,third)[->][-1]{$\times$}

\end{document}