La seconda equazione prova così.
Questo nel preambolo
`\newenvironment{sistema}%
{\left\lbrace\begin{array}{@{}l@{}}}%
{\end{array}\right.} `
e la tua equazione
`
\begin{equation}\label{eq:SYSTEM}
(S')=
\begin{sistema}
\dfrac{dM}{dx}=M\mbox{\footnotesize$\left[\Phi_{M,A}(M)\dfrac{A'(x)}{A(x)} + \Phi_{M,f}(M)f_d\dfrac{P(x)}{8A(x)} + \Phi_{M,Q}(M)\dfrac{h_0'(x)}{h_0(x)}\right]$} \\
\\
\dfrac{dp}{dx}=p\mbox{\footnotesize$\left[ \Phi_{p,A}(M)\dfrac{A'(x)}{A(x)} + \Phi_{p,f}(M) f_d\dfrac{P(x)}{8A(x)} + \Phi_{p,Q}(M)\dfrac{h_0'(x)}{h_0(x)}\right]$} \\
\\
\dfrac{dT}{dx}=T\mbox{\footnotesize$\left[\Phi_{T,A}(M)\dfrac{A'(x)}{A(x)} + \Phi_{T,f}(M) f_d \dfrac{P(x)}{8A(x)} + \Phi_{T,Q}(M) \dfrac{h_0'(x)}{h_0(x)}\right]$}
\end{sistema}
\end{equation} `
o cases al posto di sistema. Prova.
Ciao
Andrea