调整一些显示方程的垂直间距

Question

我正在使用该包changebar在我的文档中设置几个环境。

当我结束显示方程式的环境时，更改栏会延伸到方程式的太远。我希望更改栏停止在显示方程式的基线处。

我的 MWE......

\documentclass{article}
\usepackage{amsmath}
\usepackage[leftbars]{changebar}
\setlength{\changebarsep}{-20pt}
\setlength{\changebarwidth}{2pt}
\newcounter{example}
\newenvironment{exampl}
    {
        \par\vspace{10pt}
        \begin{enumerate}
            \item[]
                \refstepcounter{example}
                \noindent
                \cbstart
                    \textbf{Example \theexample} %
    }
    {
                \cbend
        \end{enumerate}
    }
\begin{document}
In Example 1 below, the bar extends to the baseline of the ending text.
\begin{exampl}
The quadratic formula
\[c^2=a^2+b^2\]
generalizes to the Law of Cosines
\[c^2=a^2+b^2-2ab\cos C\ .\]
This generalization shows how to use the Pythagorean Theorem with oblique triangles; that is, with non-right triangles.
\end{exampl}

However, Example 2 ends with a display equation and the bar extends past the baseline of the equation.
\begin{exampl}
The quadratic formula
\[c^2=a^2+b^2\]
generalizes to the Law of Cosines
\[c^2=a^2+b^2-2ab\cos C\ .\]
\end{exampl}

How can I get the bar to stop at the baseline of the display equation in Example 2?
\end{document}

我已尝试调整\belowdisplayskip，但我不知道如何仅影响我的环境末尾的显示方程。

我感谢任何帮助和评论。

Answer 1

这个答案专门针对此评论。首先，让我展示一些代码和一张图片。

代码和图片

\documentclass{article}
\usepackage{amsmath}
\usepackage[margin=2.5in]{geometry}
\usepackage[leftbars]{changebar}


\setlength{\changebarwidth}{2pt}
\setlength{\changebarsep}{0pt}
\setlength{\parindent}{0pt}

\begin{document}

\cbstart
First we wrap the entire paragraph, including the math in the changebar. 
\[ E = mc^2\]
Some text. 
\cbend

Next, we show what happens if we wrap an equation by itself in change bar. 

\cbstart
\[ E = mc^2\]
\cbend

In the next example the changebar commands are issued from \emph{within} the display math mode. 

\[ \cbstart E = mc^2 \cbend \]

In this example, we wrap the equations in some text (as it would appear within a paragraph), but only issue the changebar commands immediately before and immediately after the equation. 
\cbstart
\[ E = mc^2 \]
\cbend
some more text.

In this final example, we see what happens if we enter vertical mode before passing control to the changebar end command. 
\cbstart
\[ E  =mc^2 \]\par
\cbend

Some more text so the spacing can be more clearly seen. Just extend this a little bit more.  

\end{document}

讨论

关键在于\cbstart和\cbend命令在垂直和水平模式下的行为不同。在垂直模式下\cbend标记垂直位置，仅此而已。但在水平模式下，它会换行\vadjust，这样当前行就会先得到处理，然后再放下标记。这样做的问题是，在一个空行上，\cbend和\leavevmode\cbend的行为不同：后者将首先键入一个空白行，然后结束更改栏，从而产生一行额外的空间。

回答你的问题

如果您希望它与baseline显示的方程式齐平，您无法做到这一点，至少不能像在环境中包装事物那样做到这一点。要访问显示的方程式基线的垂直位置，您实际上必须将其放置\cbend在最终显示的方程式内。我想可能有办法做到这一点（类似于\qedhere证明环境，我期望），但这对我来说太难了。

但是，如果您对仅删除上述讨论中产生的额外空间（实际上是由于\cbend设置了额外的空白行）的解决方案感到满意，那么您可以做的是确保\cbend在垂直模式（而不是水平模式）下调用。执行此操作的命令是\par。

因此，您可以做的是在环境定义中替换对的调用，\cbend这样\par\cbend可以删除一些（但不是全部）额外的空间。

Answer 2

为什么要使用changebar？如果你要生成示例列表，使用mdframed+似乎更容易ntheorem。请参阅mdframed了解使用信息。例如：

\documentclass{article}
\usepackage{amsmath}
\usepackage{ntheorem}
\usepackage[xcolor,tikz]{mdframed}

\theoremstyle{plain}
\theorembodyfont{\upshape}
\newmdtheoremenv[%
    innertopmargin=0pt,% make the frame "tight"
    innerbottommargin=0pt,%
    rightline=false,% Kill all the lines except the left one
    topline=false,%
    bottomline=false,%
    linecolor=gray,%
    linewidth=4pt,%
    leftmargin=10pt%
    ntheorem = true% since we are using ntheorem to configure the style
        ]{exampl}{Example}


\begin{document}
In Example 1 below, the bar extends to the baseline of the ending text.
\begin{exampl}
The quadratic formula
\[c^2=a^2+b^2\]
generalizes to the Law of Cosines
\[c^2=a^2+b^2-2ab\cos C\ .\]
This generalization shows how to use the Pythagorean Theorem with oblique triangles; that is, with non-right triangles.
\end{exampl}

However, Example 2 ends with a display equation and the bar extends past the baseline of the equation.
\begin{exampl}
The quadratic formula
\[c^2=a^2+b^2\]
generalizes to the Law of Cosines
\[c^2=a^2+b^2-2ab\cos C\ .\]%
\end{exampl}

How can I get the bar to stop at the baseline of the display equation in Example 2?
\end{document}