答案1
\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\begin{document}
\[
\raisebox{-.5pt}{$i^{\textup {th}}\, \textup{entry} \longrightarrow$}
\begin{pmatrix}
a_1\\
\vdots\\
a_i\\
\vdots\\
a_n
\end{pmatrix}
\begin{matrix}
\\
\longleftarrow j^{\textup {th}}\, \textup{entry} \\
\\
\hspace{-1cm}\longrightarrow =0 \\
\end{matrix}
\]
\end{document}