\documentclass{report}
\usepackage{amsmath}
\begin{document}
Some text
\begin{align}\begin{split}
\frac{1}{8}&=\frac{1}{4}+\frac{1}{4}\\
&=\left[\frac{\dfrac{1}{2}}{2^1}\right.\\
&\left.+\frac{1}{4}\right]
\end{split}\end{align}
Some text
\end{document}