\documentclass[12pt]{article} \newcommand{\D}{\displaystyle} \begin{document} \[ a_0 + \frac{1\hfill}{\D a_1 + \frac{1\hfill}{\D a_2 + \frac{1\hfill}{\D a_3 + \frac{1\hfill}{a_4}}}} \] \end{document}