\documentclass{jarticle}
\usepackage{amsmath,ulem}

\newcommand{\bs}{\texttt{\symbol{'134}}}
\newcommand{\Bs}{\texttt{\Large\symbol{'134}}}
\newcommand\cancel{\bgroup \markoverwith{\Bs}\ULon}
\newcommand\Cancel{\bgroup \markoverwith{\texttt{\Large{/}}}\ULon}
\newcommand{\cmd}[1]{\texttt{\def\{{\char`\{}\def\}{\char`\}}\bs#1}}

\begin{document}

\begin{align*}
& \sum_{k=1}^n [1/k - 1/(k+1)] \\
&= [1-1/2]+[1/2-1/3]+\cdots+[1/n-1/(n+1)] \\
&= [1-\xout{12}]+[\xout{1/2}-\xout{1/3}]+[\xout{ 1/3}-\xout{ 1/4}]\cdots+[\xout{1/n}-1/(n+1)]  \\
& \qquad \qquad \text{(\cmd{xout}を利用)} \\
&= [1-\cancel{ 1/2}]+[\cancel{1/2}-\cancel{1/3}]+[\cancel{ 1/3}-\cancel{ 1/4}]\cdots+[\cancel{1/n}-1/(n+1)]  \\
& \qquad \qquad \text{(\cmd{cancel}を利用)} \\
&= [1-\Cancel{1/2}]+[\Cancel{1/2}-\Cancel{1/3}]+[\Cancel{ 1/3}-\Cancel{ 1/4}]\cdots+[\Cancel{1/n}-1/(n+1)]  \\
& \qquad \qquad \text{(\cmd{Cancel}を利用)} \\
&=1-1/(n+1)
\end{align*}

$$
\cancel{\frac {22222}{88888}} 
$$

\end{document}
