I want a few formulae left align (but not all in formulae in the document) and use flalign following How to align a set of multiline equations
but the result shows the formula in the center. Why?
\documentclass{article}
\usepackage{amsmath}
\begin{document}
the formula:
\noindent A \hfill Z
\begin{flalign}
1 \in N
\end{flalign}
\noindent A \hfill Z
more text
\end{document}

flinflaligndoesn't mean "flushleft", but rather "fill line" (or "full length"?). See e.g. https://tex.stackexchange.com/q/365059/82917. – campa Jun 15 '18 at 07:51[fleqn]globally (inusepackage[fleqn]{amsmath}); I would like to have a few formulae left alginged, but not set[fleqn]for all). How? – user855443 Jun 15 '18 at 08:22fleqnenvironment (defined by packagenccmath). See an examplein my answer to this question. Furthermore, the indent of the equations is a optional argument for the environment (no indent by default). – Bernard Jun 15 '18 at 08:51