I want to make it easier to see how a fraction has been expanded. I used \hfill to separate the expanded parts, but the "1s" are not in the center anymore.
How can I achieve this?
&= \frac{1}{x-y} - \frac{1}{y-x} \\
&= \frac{1\hfill*(y-x)}{(x-y)*(y-x)} - \frac{1\hfill*(x-y)}{(y-x)*(x-y)} \\
#edit: complete code for compiling
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
&= \frac{1}{x-y} - \frac{1}{y-x} \\
&= \frac{1\hfill*(y-x)}{(x-y)*(y-x)} - \frac{1\hfill*(x-y)}{(y-x)*(x-y)} \\
\end{align*}
\end{document}