aligning a multiline formula with the bullet of itemize

Question

I would like to use a multiline equation into an itemized list, but such that the bullet of the list is aligned with the first line of my equation.

The current code I use is like this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{itemize}
\item $Q_x= Q' \cup D'$
\item \begin{align*}
    \delta_x &= \left\{ \sum_{i=1}^j \ldots\text{~some big formula that spans the entire line~}\ldots\right\} \\
             &\cup  \left\{ \sum_{i=1}^j \ldots\text{~some other big formula that spans the entire line~}\ldots \right\} \\
  \end{align*}
\item $q^0_x = (q_0, \{s_0\})$
\end{itemize}
\end{document}

But this yields the following output:

I have also tried to use

\item $\displaystyle\begin{aligned}...\end{aligned}$

But in this case the bullet of the list appears between the two lines:

Any idea how to align the bullet with the first line?

Answer 1

I would definitely go the second way with $\displaystyle: I find that it looks strange to have something centered after a bullet. To get the vertical alignment correct, just add [t] after \begin{aligned}, where "t" means "top". (See also the top of page 8 of the User's guide for the amsmath Package.)