\begin{align*}
V(G_i)\in G_1\cup (G_2\cup G_3),~\text{ where}~ V(G_i)~\text{ is vertex set of}~ G_i.\\
V(G_i)\in G_1\vee (V(G_i)\in G_2\vee V(G_i)\vee G_3)\\
& \iff (V(G_i)\in G_1\vee V(G_i)\in G_2)\vee V(G_i)\in G_3)\\
& \iff V(G_i)\in (G_1\cup G_2)\cup G_3\\
& \iff G_{1}\cup (G_{2}\cup G_{3})\\
& =(G_{1}\cup G_{2})\cup G_{3}.
&
\end{align*}.
如何正确地将此文本左对齐或居中对齐？
答案1
multline*和环境的组合aligned也许可以完成这项工作：
\documentclass{article}
\usepackage{amsmath} % for 'multline*' and 'aligned' environments
\begin{document}
\noindent
$V(G_i)\in G_1\cup (G_2\cup G_3)$, where $V(G_i)$ is vertex set of $G_i$.
\begin{multline*}
V(G_i)\in G_1\vee{} (V(G_i)\in G_2\vee V(G_i)\vee G_3)\\
\begin{aligned}
\iff &(V(G_i)\in G_1\vee V(G_i)\in G_2)\vee V(G_i)\in G_3)\\
\iff &V(G_i)\in (G_1\cup G_2)\cup G_3\\
\iff &G_{1}\cup (G_{2}\cup G_{3})\\
{}={}&(G_{1}\cup G_{2})\cup G_{3}.
\end{aligned}
\end{multline*}
\end{document}