分式中的约分

[shaodongtang]

cancel 宏包. mathmode 定义
\newcommand\hcancel[2][black]{\setbox0=\hbox{#2}\rlap{\raisebox{.45\ht0}{\textcolor{#1}{\rule{\wd0}{1pt}}}}#2}
例如：
$f(x)=\dfrac{\left(x^2+1\right)\cancel{(x-1)}}{\cancel{(x-1)}(x+1)}$\[0.5cm]
$\bcancel{3}\qquad\bcancel{1234567}$\[0.5cm]
$\xcancel{3}\qquad\xcancel{1234567}$\[0.5cm]
$\hcancel{3}\qquad\hcancel[red]{1234567}$

也可以用tikz画（Miscellaneous Shapes）p735
\usetikzlibrary{shapes.misc}
\tikz[baseline]\node[strike out,draw,anchor=text]{me};
\tikz[baseline] \node [cross out,draw,anchor=text] {me};
\tikz \node [cross out,draw=red] at (1.5,1) {cross out};