第一个例子

Question

\dfrac{\cancel{4} \times \cancel{5} \times \cancel{3}}{\cancel{2} \times \cancel{10}}

Answer 1

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我会使用(tikz) 库中的tikz和 来执行此操作。该库也可用于控制位置（可以根据需要进行改进）。matrix of math nodesmatrixcalc

类似下面的内容：

\documentclass[tikz,border=2mm]{standalone}
\usetikzlibrary{calc,matrix}

\begin{document}
\begin{tikzpicture}[line cap=round]
  \matrix(M)[blue,matrix of math nodes,
              row 3/.style={black},
              row 4/.style={black},
             ]
  {%
    1  &        &        &        &   \\
    2  &        & 1      &        &   \\
    4  & \times & 5      & \times & 3 \\
    {} & 2      & \times & 10     & {}\\
       & 1      &        &  2     &   \\
       &        &        &  1     &   \\
  };
  \draw (M-3-1.south west) -- (M-3-5.south east);
  \foreach\i/\j in {2/1,3/1,3/3,4/2,4/4,5/4}
    \draw[blue] ($(M-\i-\j.north west)-(0,0.1)$) -- ($(M-\i-\j.south east)+(0,0.1)$);
  \node at ($(M-3-5)!0.5!(M-4-5)+( 0.2,0.07)$) [right] {$\mathstrut=-3$};
  \node at ($(M-3-1)!0.5!(M-4-1)+(-0.2,0.07)$) [left]  {$\mathstrut-$};
\end{tikzpicture}
\end{document}

编辑：按照 OP 的建议，添加了减号。

Answer 2

第一个例子

\documentclass[preview,border=12pt]{standalone}% please change it to your own class, for example article, book, etc
\usepackage{amsmath}
\usepackage{cancel}

\begin{document}
$
\begin{aligned}
t &=\frac{\bcancel{a}\times b}{\bcancel{a}} \\
  &= b
\end{aligned}
$
\end{document}

第二示例

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{cancel}

\begin{document}
Given $f(x)$. Find its derivative with respect to $x$.
\begin{align*}
    f'(x) &= \frac{\cancel{\mathrm{d}}f(x)}{\cancel{\mathrm{d}} x} \\
          &= \frac{f(x)}{x}
\end{align*}
\end{document}