答案1
四种方法。第一种方法我想不出如何描述。
\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\Var}{Var}
\begin{document}
\begin{align}
\Var(\bar{x})&=\Var\left(\begin{matrix}\dfrac{\displaystyle\sum_{i=1}^{n} x_i}{n}\end{matrix}\right)
\tag{---}
\\[2ex]
\Var(\bar{x})&=\Var\left(\begin{matrix}\dfrac{\sum\limits_{i=1}^{n} x_i}{n}\end{matrix}\right)
\tag{ugly}
\\[2ex]
\Var(\bar{x})&=\Var\left(\frac{\sum_{i=1}^{n} x_i}{n}\right)
\tag{bad}
\\[2ex]
\Var(\bar{x})&=\Var\biggl(\frac{1}{n}\sum_{i=1}^{n} x_i\biggr)
\tag{good}
\end{align}
\end{document}
答案2
我建议使用以下解决方案之一 pmatrix
:
\documentclass{article}
\usepackage{mathtools}
\DeclareMathOperator{\Var}{Var}
\begin{document}
\[
\Var(\overline{x}) = \Var\begin{pmatrix}\dfrac{\displaystyle ∑_{i=1}ⁿ X_{i}}{n}\end{pmatrix}
\]
\[
\Var(\overline{x}) = \Var\begin{pmatrix}\dfrac{\displaystyle \sum\nolimits_{i=1}ⁿ X_{i}}{n}\end{pmatrix}
\]
\end{document}