\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}