答案1
\documentclass{article}
\begin{document}
\begin{equation}
h_n\biggl[f_X(0)\underbrace{\int_0^{+\infty}K(z)^2dz}_{\equiv\beta} +o_p(1)\biggr]
\end{equation}
\end{document}
\documentclass{article}
\begin{document}
\begin{equation}
h_n\biggl[f_X(0)\underbrace{\int_0^{+\infty}K(z)^2dz}_{\equiv\beta} +o_p(1)\biggr]
\end{equation}
\end{document}