A me non sfonda nemmeno con 12pt e itemize: prova il mio codice:
`
\documentclass[12pt,draft]{book}
\usepackage{amsmath}
\newenvironment{sistema}%
{\left\lbrace\begin{array}{@{}l@{}}}%
{\end{array}\right.}
\begin{document}
\begin{itemize}
\item Ecco il primo sistema di equazioni:
\begin{equation*}
\begin{sistema}
\biggl( \dfrac{1}{3} \dfrac{H}{E I_p} + \dfrac{1}{3} \dfrac{L}{E I_t} \biggr) X_A – \dfrac{1}{6} \dfrac{L}{E I_t} X_B – \dfrac{1}{6} \dfrac{H}{E I_p} X_C – \dfrac{\eta}{H} + \dfrac{P H^2}{16 E I_p} = 0 \\[2.5ex]
\dfrac{1}{6} \dfrac{L}{E I_t} X_A – \biggl( \dfrac{1}{3} \dfrac{L}{E I_t} + \dfrac{1}{3} \dfrac{H}{E I_p} \biggr) X_B + \dfrac{1}{6} \dfrac{H}{E I_p} X_D + \dfrac{\eta}{H} – \dfrac{p H^2}{16 E I_p} = 0 \\[2.5ex]
\dfrac{1}{6} \dfrac{H}{E I_p} X_A – \dfrac{1}{3} \dfrac{H}{E I_p} X_C + \dfrac{\eta}{H} + \dfrac{P H^2}{16 E I_p} = 0 \\[2.5ex]
– \dfrac{1}{6} \dfrac{H}{E I_p} X_B + \dfrac{1}{3} \dfrac{H}{E I_p} X_D – \dfrac{\eta}{H} – \dfrac{P H^2}{16 E I_p} = 0 \\[2.5ex]
-2 F – P + \dfrac{X_A + X_B + X_C + X_D}{H} = 0 \\[1.0ex]
\end{sistema}
\end{equation*}
\end{itemize}
\end{document}
`