\documentclass[fleqn]{article}
\usepackage{amssymb,amsmath,amsfonts}
\usepackage{latexsym}

\parindent=0pt
\parskip=25pt

\pagestyle{empty}

\begin{document}

\huge

\begin{equation} %Montrer exemple \, \; \!
x^{2} \geq 0  \textrm{pour tout} x\in \mathbf{R}
\label{eq:textrm}
\end{equation}

\begin{equation}
x^{2} \geq 0  \textrm{ pour tout } x\in \mathbf{R}
\label{eq:textrm2}
\end{equation}

\bigskip

Ici, on voit du texte normal en roman.  {\sffamily Ensuite, on voit du texte en sans-serif (\c ca ressemble beaucoup \`a du arial en Word).}


{\sffamily Bla-bla-bla
\begin{equation} d^{\textrm{essai}} \qquad d^{\text{essai}}
\label{eq:text}\end{equation}
bla-bla-bla}


\newpage
\large
\subsection*{\Large intertext}


\hrule

\begin{align}
a+b&=c &&\forall i\leq I\tag{AMS} \label{eq:align5}
\end{align}
subject to:
\begin{align}
a^a+b^a&\leq c^a - d^g - e^4, &&\forall i\leq I \label{eq:align9}\\
a^1+b^2+c^a - d^g - e^4&= c^3.\nonumber
\end{align}

\hrule
\begin{align}
a+b&=c &&\forall i\leq I\tag{AMS} \label{eq:align7}\\
%
\intertext{subject to:}
%
a^a+b^a&\leq c^a - d^g - e^4, &&\forall i\leq I\label{eq:align8}\\
a^1+b^2c^a - d^g - e^4&= c^3.\nonumber
\end{align}

\hrule
\begin{align}\label{E:mm5}
      h(x) &= \int \left(
                         \frac{ f(x) + g(x) }
                              {1 + f^{2}(x)} +
                         \frac{1 + f(x)g(x)}
                              { \sqrt{1 - \sin x} }
                   \right) \, dx\\
   \intertext{The reader may find the following form easier to
    read:}
      &= \int \frac{1 + f(x)}
                   {1 + g(x)}
         \, dx - 2 (x - 2) \notag
\end{align}

\hrule
Define
\begin{align*}
\hat x_0   &= [x   \,|\, y_0 ] = \bar x   + \Theta_0 \Sigma_{00}^{-1} (y_0 - \bar y_0) , \\
\intertext{and for $i, j \in N$,}
\hat y_i &= [y_i \,|\, y_0 ] = \bar y_i + \Sigma_{i0} \Sigma_{00}^{-1} (y_0 - \bar y_0) , \\
\hat \Theta_i &= (x, y_i \,|\, y_0) = \Theta_i - \Theta_0 \Sigma_{00}^{-1} \Sigma_{0i}, \\
\hat \Sigma_{ij} &= (y_i, y_j \,|\, y_0) = \Sigma_{ij} - \Sigma_{i0} \Sigma_{00}^{-1} \Sigma_{0j}.
\end{align*}


\end{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass[fleqn]{article}
\usepackage{amssymb,amsmath,amsfonts}
\usepackage{latexsym}

\parindent=0pt
\parskip=25pt

\pagestyle{empty}


	
	\huge
	
	\begin{equation} %Montrer exemple \, \; \!
	x^{2} \geq 0  \textrm{pour tout} x\in \mathbf{R}
	\label{eq:textrm}
	\end{equation}
	
	\begin{equation}
	x^{2} \geq 0  \textrm{ pour tout } x\in \mathbf{R}
	\label{eq:textrm2}
	\end{equation}
	
	\bigskip
	
	Ici, on voit du texte normal en roman.  {\sffamily Ensuite, on voit du texte en sans-serif (\c ca ressemble beaucoup \`a du arial en Word).}
	
	
	{\sffamily Bla-bla-bla
		\begin{equation} d^{\textrm{essai}} \qquad d^{\text{essai}}
		\label{eq:text}\end{equation}
		bla-bla-bla}
	
	
	\newpage
	\large
	\subsection*{\Large intertext}
	
	
	\hrule
	
	\begin{align}
	a+b&=c &&\forall i\leq I\tag{AMS} \label{eq:align5}
	\end{align}
	subject to:
	\begin{align}
	a^a+b^a&\leq c^a - d^g - e^4, &&\forall i\leq I \label{eq:align9}\\
	a^1+b^2+c^a - d^g - e^4&= c^3.\nonumber
	\end{align}
	
	\hrule
	\begin{align}
	a+b&=c &&\forall i\leq I\tag{AMS} \label{eq:align7}\\
	%
	\intertext{subject to:}
	%
	a^a+b^a&\leq c^a - d^g - e^4, &&\forall i\leq I\label{eq:align8}\\
	a^1+b^2c^a - d^g - e^4&= c^3.\nonumber
	\end{align}
	
	\hrule
	\begin{align}\label{E:mm5}
	h(x) &= \int \left(
	\frac{ f(x) + g(x) }
	{1 + f^{2}(x)} +
	\frac{1 + f(x)g(x)}
	{ \sqrt{1 - \sin x} }
	\right) \, dx\\
	\intertext{The reader may find the following form easier to
		read:}
	&= \int \frac{1 + f(x)}
	{1 + g(x)}
	\, dx - 2 (x - 2) \notag
	\end{align}
	
	\hrule
	Define
	\begin{align*}
	\hat x_0   &= [x   \,|\, y_0 ] = \bar x   + \Theta_0 \Sigma_{00}^{-1} (y_0 - \bar y_0) , \\
	\intertext{and for $i, j \in N$,}
	\hat y_i &= [y_i \,|\, y_0 ] = \bar y_i + \Sigma_{i0} \Sigma_{00}^{-1} (y_0 - \bar y_0) , \\
	\hat \Theta_i &= (x, y_i \,|\, y_0) = \Theta_i - \Theta_0 \Sigma_{00}^{-1} \Sigma_{0i}, \\
	\hat \Sigma_{ij} &= (y_i, y_j \,|\, y_0) = \Sigma_{ij} - \Sigma_{i0} \Sigma_{00}^{-1} \Sigma_{0j}.
	\end{align*}
	
	

