edit:
- it seems that on the answer forgot as op as well me :-(.
- today i recognized, that the loaded mwe is not complete ... and that CarLaTeX ask me in her comment (which seems that i overlooked). i'm very sorry for this.
- now i add missing part of mwe: :
\begin{document}
, \begin{fullwidth}
and definitions of used lengths,
- beside this i add
showframe
package to for show page layout (which had to be removed in real document)
now mwe works as expected (tested).

\documentclass{tufte-handout}
\usepackage{multirow}
\usepackage[italian]{babel}
\usepackage[utf8]{inputenc}
\usepackage{booktabs,tabularx}
\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}
\newlength\lengtha % width of two-columm multicolumn
\newlength\lengthb % width of four-columm multicolumn
\newlength\lengthc % width of six-columm multicolumn
%
\setlength\aboverulesep{0pt}
\setlength\belowrulesep{0pt}
%-------------------------------- show page layout, only for test
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%
\begin{document}
\begin{fullwidth}
\setlength\lengtha{\dimexpr.2\hsize+2\tabcolsep-2\arrayrulewidth\relax}
\setlength\lengthb{\dimexpr.4\hsize+6\tabcolsep-4\arrayrulewidth\relax}
\setlength\lengthc{\dimexpr.6\hsize+10\tabcolsep-6\arrayrulewidth\relax}
\begin{tabularx}{\linewidth}{|*{10}{C|} }
\toprule
\multicolumn{4}{|P{\lengthb}|}{Elemento}
& \multicolumn{6}{P{\lengthc}|}{FUNZIONI TRASCENDENTI} \\
\midrule
\multicolumn{2}{|P{\lengtha}|}{?}
& \multicolumn{2}{P{\lengtha}|}{?}
& \multicolumn{2}{P{\lengtha}|}{LOGARITMICHE}
& \multicolumn{2}{P{\lengtha}|}{ESPONENZIALI}
& \multicolumn{2}{P{\lengtha}|}{LOGARITMICHE} \\
\midrule
& & & & & & & & & \\
\bottomrule
\end{tabularx}
\end{fullwidth}
\end{document}
Actually I reproduce showed picture of table and text from your MWE place as you can see from above image. I'm not sure if I guess correctly what you like to obtain.