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## 编译环境
操作系统
* [x]Windows 7/8/10
* [ ] macOS
* [ ] Linux
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Tex发行版
* [x]TexLive 2020
* [ ] MikTeX `版本号`
* [ ] CTeX
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## 我的问题
在此输入你遇到的问题...
如图，这一段公式我打了\\强制换行，但是pdf的输出结果没反应，这是为什么呢？代码如下：
```c
\documentclass{article}%article,book,report,letter
\title{hhh}%文档编辑标题
\author{student}%文档编辑作者
\date{\today}%文档编辑时间
\begin{document}%文件类
\maketitle%输出本文标题
1.
(a)
$$
\nabla \left( fg \right) =\left( \frac{\partial \left( fg \right)}{\partial x_1},\frac{\partial \left( fg \right)}{\partial x_2},...,\frac{\partial \left( fg \right)}{\partial x_n} \right)
\\
\nabla \left( fg \right) =\left( g\frac{\partial f}{\partial x_1}+f\frac{\partial g}{\partial x_1},g\frac{\partial f}{\partial x_2}+f\frac{\partial g}{\partial x_2},...,g\frac{\partial f}{\partial x_n}+f\frac{\partial g}{\partial x_n} \right)
\\
\nabla \left( \frac{f}{g} \right) =\left( \frac{\partial \left( \frac{f}{g} \right)}{\partial x_1},\frac{\partial \left( \frac{f}{g} \right)}{\partial x_2},...,\frac{\partial \left( \frac{f}{g} \right)}{\partial x_n} \right)
\\
\nabla \left( \frac{f}{g} \right) =\left( \frac{\frac{\partial f}{\partial x_1}g-\frac{\partial g}{\partial x_1}f}{g^2},\frac{\frac{\partial f}{\partial x_2}g-\frac{\partial g}{\partial x_2}f}{g^2},...,\frac{\frac{\partial f}{\partial x_n}g-\frac{\partial g}{\partial x_n}f}{g^2} \right)
$$
(b)
$
\nabla \cdot \left( f\nabla g \right) =\nabla f\cdot \nabla g+f\varDelta g
\\
\nabla \cdot \left( f\nabla g \right) =\left( \frac{\partial}{\partial x_1},\frac{\partial}{\partial x_2},...,\frac{\partial}{\partial x_n} \right) \cdot \left( f\frac{\partial g}{\partial x_1},f\frac{\partial g}{\partial x_2},...,f\frac{\partial g}{\partial x_n} \right)
\\
=\frac{\partial}{\partial x_1}f\frac{\partial g}{\partial x_1}+\frac{\partial}{\partial x_2}f\frac{\partial g}{\partial x_2}+...+\frac{\partial}{\partial x_n}f\frac{\partial g}{\partial x_n}\\=\frac{\partial f}{\partial x_1}\frac{\partial g}{\partial x_1}+f\frac{\partial ^2g}{\partial x_{1}^{2}}+\frac{\partial f}{\partial x_2}\frac{\partial g}{\partial x_2}+f\frac{\partial ^2g}{\partial x_{2}^{2}}+...+\frac{\partial f}{\partial x_n}\frac{\partial g}{\partial x_n}+f\frac{\partial ^2g}{\partial x_{n}^{2}}\\=\frac{\partial f}{\partial x_1}\frac{\partial g}{\partial x_1}+\frac{\partial f}{\partial x_2}\frac{\partial g}{\partial x_2}+...+\frac{\partial f}{\partial x_n}\frac{\partial g}{\partial x_n}+f\frac{\partial ^2g}{\partial x_{1}^{2}}+f\frac{\partial ^2g}{\partial x_{2}^{2}}+...+f\frac{\partial ^2g}{\partial x_{n}^{2}}
\\=\nabla f\cdot \nabla g+f\varDelta g$
\end{document}
```
1 回答
6
数学环境中不允许有空行（分段），行间公式无法用\\命令手动换行，排版多行公式需要用其他环境，如align,gather环境。
align环境将公式用&进行分割并对齐，会给每个公式进行编号，可以用\notag去掉某一行的编号。如果不需要按等号对齐，只需罗列公式的话，gather会是一个很好用的环境。
```c
\documentclass{article}%article,book,report,letter
\usepackage{amsmath}
\title{hhh}%文档编辑标题
\author{student}%文档编辑作者
\date{\today}%文档编辑时间
\begin{document}%文件类
\maketitle%输出本文标题
\begin{gather}
\nabla \left( fg \right) =\left( \frac{\partial \left( fg \right)}{\partial x_1},\frac{\partial \left( fg \right)}{\partial x_2},...,\frac{\partial \left( fg \right)}{\partial x_n} \right) \notag
\\
\nabla \left( fg \right) =\left( g\frac{\partial f}{\partial x_1}+f\frac{\partial g}{\partial x_1},g\frac{\partial f}{\partial x_2}+f\frac{\partial g}{\partial x_2},...,g\frac{\partial f}{\partial x_n}+f\frac{\partial g}{\partial x_n} \right) \notag
\\
\nabla \left( \frac{f}{g} \right) =\left( \frac{\partial \left( \frac{f}{g} \right)}{\partial x_1},\frac{\partial \left( \frac{f}{g} \right)}{\partial x_2},...,\frac{\partial \left( \frac{f}{g} \right)}{\partial x_n} \right) \notag
\\
\nabla \left( \frac{f}{g} \right) =\left( \frac{\frac{\partial f}{\partial x_1}g-\frac{\partial g}{\partial x_1}f}{g^2},\frac{\frac{\partial f}{\partial x_2}g-\frac{\partial g}{\partial x_2}f}{g^2},...,\frac{\frac{\partial f}{\partial x_n}g-\frac{\partial g}{\partial x_n}f}{g^2} \right) \notag
\end{gather}
\end{document}
```
-
我明白了！谢谢您！！ – 岩岫杳冥 2020-10-05 01:23 回复
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