\begin{table}[H]
\centering
\begin{tabular}{m{0pt}|m{4cm}<{\centering}|m{4cm}<{\centering}|}
\cline{2-3}
\rule{0pt}{20pt} & $\sigma$已知 & $\sigma$未知 \\ \cline{2-3}
\rule{0pt}{25pt} & $\sigma_M=\dfrac{\sigma}{\sqrt{n}}$ & $s_M=\dfrac{s}{\sqrt{n}}$ \\ \cline{2-3}
\rule{0pt}{25pt} & $z=\dfrac{M-\mu}{\sigma_M}$ & $t=\dfrac{M-\mu}{s_M}$ \\ \cline{2-3}
\end{tabular}
\end{table}
\begin{tblr}{
colspec = {|Q[c,wd=40mm]|Q[c,wd=40mm]|},
rowspec = {Q[m,ht=5mm]Q[m,ht=10mm]Q[m,ht=10mm]}
}
\hline
$\sigma$已知 & $\sigma$未知 \\
\hline
$\sigma_M=\dfrac{\sigma}{\sqrt{n}}$ & $s_M=\dfrac{s}{\sqrt{n}}$ \\
\hline
$z=\dfrac{M-\mu}{\sigma_M}$ & $t=\dfrac{M-\mu}{s_M}$ \\
\hline
\end{tblr}
\begin{tblr}
\hline
$\sigma$已知 & $\sigma$未知 \\
\hline
\SetRow{rowsep=10pt}$\sigma_M=\dfrac{\sigma}{\sqrt{n}}$ & $s_M=\dfrac{s}{\sqrt{n}}$ \\
\hline
\SetRow{rowsep=10pt}$z=\dfrac{M-\mu}{\sigma_M}$ & $t=\dfrac{M-\mu}{s_M}$ \\
\hline
\end{tblr}