bmatrix
只需在分母中使用一个环境:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
b\cdot[0;a_1,a_2,a_3,\cdots]=\cfrac{b}{a_1+\cfrac{1}{a_2+\cfrac{1}{a_3+\cdots}}}=
\cfrac{1}{\cfrac{1}{b} \begin{bmatrix} a_1+\cfrac{1}{a_2+\cfrac{1}{a_3+\cdots}} \end{bmatrix}}
\]
\end{document}