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%   appendix
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usepackage[titletoc,toc,title,page]{appendix}

addtocaptionsitalian{%
   renewcommand{appendixtocname}{Appendici}%
   renewcommand{appendixpagename}{Appendici}%
}

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part{textcolor{SandyBrown}{textsc{Appendice}}}
%chapterimage{chapter_head_9.pdf} % Chapter heading image
%chapter{Richiami di formalismo vettoriale}
appendix %%%%%<---------------------command that I have added
chapterimage{chapter_head_9.pdf} % Chapter heading image
chapter{Richiami di formalismo vettoriale}
section{Campi vettoriali, scalari, radiali, centrali e conservativi}index{Campi vettoriali, scalari, radiali, centrali e conservativi}