即使 amsmath 和 mathtool 包已包含，“align”和“flalign”仍未定义

Question

我试图将两个方程对齐在同一行上，并且它们共享相同的标签：

\begin{falign*}
\begin{equation}
    \rho(\mathbf {r'}) = -\nabla \cdot \mathbf M(\mathbf {r'})  \qquad  \text{and} \qquad  \sigma(\mathbf {r'}) = \mathbf M(\mathbf {r'}) \cdot \Hat{\mathbf{n}}
\end{equation}
\end{falign*}

它按我想要的方式显示，但我收到以下错误消息：环境 falign* 未定义如果我使用“align*”而不是“falign*”，我会收到另一个错误：“方程式在 align 内”，如果我更改环境的顺序，我会得到“方程式内对齐”。两者都给我同样的问题。此外，如果我只使用对齐方程式居中，我需要它们在左侧。

Answer 1

您有falign*，没有flalign*。无论如何，equation既不能进去flalign也不能进去align（有或没有*）。

如果你想向左平齐方程式，你想将fleqn选项传递给文档类，并且

\setlength{\mathindent}{0pt}

例子。

\documentclass[a4paper,fleqn]{article}
\usepackage{amsmath}

\setlength{\mathindent}{0pt}

\begin{document}

Some nonsense text just to see what really happens to equations;
some nonsense text just to see what really happens to equations
\begin{equation}
a=b
\end{equation}
some nonsense text just to see what really happens to equations;
some nonsense text just to see what really happens to equations
\begin{align}
a&=b+c \\
x&=u+v+w
\end{align}
some nonsense text just to see what really happens to equations;
some nonsense text just to see what really happens to equations.

\end{document}

在fl​flalign​全长，不向左齐平。

您可能希望方程式编号位于左侧。为此，请传递选项leqno，不要设置\mathindent。

\documentclass[a4paper,fleqn,leqno]{article}
\usepackage{amsmath}

\begin{document}

Some nonsense text just to see what really happens to equations;
some nonsense text just to see what really happens to equations
\begin{equation}
a=b
\end{equation}
some nonsense text just to see what really happens to equations;
some nonsense text just to see what really happens to equations
\begin{align}
a&=b+c \\
x&=u+v+w
\end{align}
some nonsense text just to see what really happens to equations;
some nonsense text just to see what really happens to equations.

\end{document}