\documentclass{article}
\begin{document}
\section{Display Math}
Begin first paragraph,
\[ a + b, \]
continuing paragraph.

Single line paragraph.

\[ a + b. \]

Another single line paragraph.

\section{Itemizations}
Begin first paragraph,
\begin{itemize}
\item A
\item B
\end{itemize}
continuing paragraph.

Single line paragraph.

\begin{itemize}
\item A
\item B
\end{itemize}

Another single line paragraph.


Multiple Para within item
\begin{itemize}
\item A

B
\item C
\end{itemize}

\end{document}