> For the complete documentation index, see [llms.txt](https://overleaf-pro.ayaka.space/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://overleaf-pro.ayaka.space/latex/zh-tw/latex-cuo-wu/06-extra-right.md).

# 多餘的 \right

當使用 \right 指令但缺少對應的 \left 時，在排版數學式時就會發生這個錯誤。本文會討論這個錯誤的常見原因，以及如何修正。

## \left 和 \right 指令簡介

該 `\left` 以及 `\right` 這些指令會排版可動態調整大小的定界符，且必須在數學內容中一起使用，格式為

$$\verb'\left'\thinspace\textit{delim}*{\thinspace\text{L}}\quad\textit{math expression}\quad \verb'\right'\thinspace\textit{delim}*{\thinspace\text{R}}$$

其中 $$\textit{delim}*{\thinspace\text{L}}$$ 以及 $$\textit{delim}*{\thinspace\text{R}}$$ 是用來包住你的 $$\textit{math expression}$$。通常， $$\textit{delim}*{\thinspace\text{L}}$$ 以及 $$\textit{delim}*{\thinspace\text{R}}$$ 是以下字元之一 `() [] | \| \{ \}` 或點號「`.`」，它可作為「[空白定界符](#using-a-blank-delimiter)」。

### 範例

以下是一個使用 `\left` 以及 `\right` 來排版完全包住分數的括號的範例：

```latex
\[
  \left(\frac{x}{y} \right)
\]
```

此程式碼會產生 $$\left(\frac{x}{y}\right)$$

如果你沒有使用 `\left` 以及 `\right` 指令來放置定界符（包括括號），那些定界符就不會完全包住數學表達式；例如，寫成

```latex
\[
  (\frac{x}{y})
\]
```

會產生這個

$$(\frac{x}{y})$$

## 此錯誤的原因與一些解決方法

### 忘記加入 \left 指令

這個錯誤通常在撰寫一個 `\right` 指令，但忘記加入對應的 `\left`，如下方範例所示：

```latex
\[
\pi =
( \int_{-\infty}^{+\infty} e^{-x^2} dx \right)
\]
```

[在此開啟 **會產生錯誤的** Overleaf 中的範例](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=Example+demonstrating+a+missing+delimiter\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%5B+%0A%5Cpi+%3D+%0A%28+%5Cint_%7B-%5Cinfty%7D%5E%7B%2B%5Cinfty%7D+e%5E%7B-x%5E2%7D+dx+%5Cright%29%0A%5C%5D%0A%5Cend%7Bdocument%7D)

這個範例會產生以下錯誤：

![一張用來示範 LaTeX 中缺少分隔符號的圖片](/files/59bec1354feb5769e7916179047052ff8e24be55)

加入缺少的 `\left` 以修正此錯誤，只需寫成

```latex
\[
\pi =
\left( \int_{-\infty}^{+\infty} e^{-x^2} dx \right)
\]
```

，即可產生以下輸出

$$\pi = \left( \int\_{-\infty}^{+\infty} e^{-x^2} dx \right)$$

### 使用空白定界符

如果你想排版一個 `\right` 僅有的定界符，請使用 ‘`.`’ 字元作為 `\left` 定界符，寫成 `\left.`，它不會排版出定界符字元。以下範例使用 `.` 一個環境中的定界符 `aligned` 環境中，該環境由 `amsmath` 套件中可用的顏色與粗細參數：

```latex
\documentclass{article}
\usepackage{amsmath} % 使用 aligned 環境所需

\begin{document}
\[
 \left.\begin{aligned}
        dF &= 0,\\
        d^{\dagger} F &= 0,
       \end{aligned}
\ \right\}
 \qquad \text{麥克斯威方程組}
\]
\end{document}
```

[在 Overleaf 中開啟此範例](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=Using+the+dot+delimiter\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cusepackage%7Bamsmath%7D+%25+Required+to+use+the+aligned+environment%0A%0A%5Cbegin%7Bdocument%7D%0A%5C%5B%0A+%5Cleft.%5Cbegin%7Baligned%7D%0A++++++++dF+%26%3D+0%2C%5C%5C%0A++++++++d%5E%7B%5Cdagger%7D+F+%26%3D+0%2C%0A+++++++%5Cend%7Baligned%7D%0A%5C+%5Cright%5C%7D%0A+%5Cqquad+%5Ctext%7BMaxwell%27s+equations%7D%0A%5C%5D%0A%5Cend%7Bdocument%7D)

這個範例會排版出以下表達式：

$$\left.\begin{aligned} dF &= 0,\d^{\dagger} F &= 0,\end{aligned}\ \right}\qquad \text{Maxwell's equations}$$


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