cancel package downward arrow

Question

Let's say that I want to cancel the denominator of a fraction and also assign him a value. Something like that \cancel{2}\cdot\dfrac{9}{\cancelto{2}{4}}. The problem here is that the fraction doesn't look good at all after that. So I need a cancelto command that creates a downward arrow. Is there any way to do it?

Answer 1

To change the arrow direction you need to rewrite most of the internals of the cancel package. I would NOT recommend that, because it is very complicated code. May be you can try some other options such as

\begin{equation}
\bcancel{2}\cdot\dfrac{9}{{\bcancel{4}}^2_{}}
\end{equation}

or

\begin{equation}
\cancel{2}\cdot\dfrac{9}{{\cancel{4}\rule{0pt}{2.ex}}^{\;2}_{}}
\end{equation}

Answer 2

With the help of TikZ you can define a command to cancel an expression using a downward arrow:

\documentclass{scrartcl}
\usepackage{amsmath}
\usepackage{cancel}
\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand*\canc[1]{%
  \mathchoice
    {\scriptstyle#1}
    {\scriptstyle#1}
    {\scriptscriptstyle#1}
    {\scriptscriptstyle#1}
}

\newcommand*\Dcancelto[2][0]{%
  \kern9pt%
  \begin{tikzpicture}[baseline=(current bounding box.center).anchor=west]
    \node[anchor=east,inner sep=2pt] (a) {#2};
    \draw[->] ($(a.north west)+(1pt,-2pt)$) -- ($(a.south east)+(0pt,2pt)$) node at ($(a.south east)+(4pt,1pt)$) {$\canc{#1}$};
\end{tikzpicture}
}

\begin{document} 

\[ \Dcancelto[1]{7} \]

\[ \frac{1}{\Dcancelto[2]{4}} \]

\[ \frac{89}{\Dcancelto[0]{4367}} \]

\[ \cancel{2}\cdot\dfrac{9}{\Dcancelto[2]{4}} \]

\end{document}

Here's the result: