
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{mathtools}
\pagestyle{empty}
\begin{document}
\begin{align}
  A//1&=N//0(\lambda;\Omega')-\phi(\lambda;\Omega'),\\
  A_2&=\phi(\lambda;\Omega')-\phi(\lambda;\Omega),\\
  \intertext{et} 
  A_3&=
      \mathcal{N}(\lambda;\omega).
\end{align}
\end{document}
