\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amssymb,amsfonts,tikz}
\usepackage[left=3.3cm,right=1cm,top=1.5cm,bottom=2.5cm]{geometry}
\definecolor{col1}{RGB}{243,111,36}
\definecolor{col2}{RGB}{0,182,187}
\definecolor{col3}{RGB}{240,225,225}
\definecolor{col}{RGB}{221,10,138}
\definecolor{col5}{RGB}{0,182,189}
\everymath{\displaystyle}
\usetikzlibrary{shadows}
\pagestyle{empty}
\usepackage{pifont,fourier}
\usetikzlibrary{shapes.arrows,calc}
\linespread{1.3}
\usepackage{amsmath,amsfonts,amssymb,tikz,enumitem}
%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{eso-pic}
\AddToShipoutPictureBG{
\begin{tikzpicture}[remember picture,overlay]
\fill[col1,rotate=-50](current page.north west)to[out=-1,in=140]([yshift=-7cm]current page.north east)--(current page.north east)--cycle;
\fill[col1,rotate=-130,general shadow={fill=black!20,shadow xshift=-5pt}](current page.south east)--([yshift=-4cm]current page.south east)to[out=-1,in=140](current page.south west)--cycle;
\fill[col1!75!black,,rotate=-50](current page.north west)to[out=0,in=140]([yshift=-6.7cm]current page.north east)--([yshift=-6cm]current page.north east)to[out=140,in=0](current page.north west);
\draw[line width=3pt,col1]([xshift=0cm,yshift=-9cm]current page.north west)--([xshift=0cm,yshift=-9cm]current page.north east);
\fill[line width=3pt,draw=white,fill=col1]([xshift=2cm,yshift=-9cm]current page.north west)--++(1,1)--++(1,-1)--++(-1,-1)--cycle;
\fill[line width=2pt,draw=white,fill=col1]([xshift=1.5cm,yshift=-9cm]current page.north west)--++(1,1)--++(1,-1)--++(-1,-1)--cycle;
\fill[line width=1pt,draw=white,fill=col1]([xshift=1cm,yshift=-9cm]current page.north west)--++(1,1)--++(1,-1)--++(-1,-1)--cycle;
\fill[line width=0.5pt,draw=white,fill=col1]([xshift=0.5cm,yshift=-9cm]current page.north west)--++(1,1)--++(1,-1)--++(-1,-1)--cycle;
\fill[line width=0.1pt,draw=white,fill=col1]([xshift=0cm,yshift=-9cm]current page.north west)--++(1,1)--++(1,-1)--++(-1,-1)--cycle;
\fill[line width=0.05pt,draw=white,fill=col1]([xshift=-0.5cm,yshift=-9cm]current page.north west)--++(1,1)--++(1,-1)--++(-1,-1)--cycle;
\draw[dashed,line width=2pt,col1]([xshift=3cm,yshift=-10cm]current page.north west)--([xshift=3cm]current page.south west);
\node[scale=2,rotate=0] at([xshift=4cm,yshift=-3.3cm]current page.north west){\color{col1}{\large \textbf{\textit{Chapitre 3}}}};
\node[anchor=west,scale=4] at([xshift=-7cm,yshift=-6cm]current page.north){\color{col1!75!black}{ \textbf{\textit{ Nombres Complexes}}}};
\node[circle,fill=white,minimum size=2mm] at([xshift=-3cm,yshift=1cm]current page.south east){\color{red}{\Large \textbf{{ \thepage}}}};
\node[anchor=west] at([xshift=4cm,yshift=-8.5cm]current page.north west){\color{col1}{\Large \textbf{{Résumé de cours}}}};
\end{tikzpicture}
}
%%%%%%%%%%%%%
\usepackage{tcolorbox}
\tcbuselibrary{skins,breakable}
\tcbset{nobeforeafter, colbacktitle=white, colback=white, coltitle=black, enhanced, frame code={\path[tcb fill frame] (frame.south west)--([yshift=-5mm]frame.north west)
--([xshift=5mm]frame.north west)--(frame.north east)--([yshift=-5mm]frame.north east)
--([yshift=5mm]frame.south east)--([xshift=-5mm]frame.south east)--cycle;},colframe=col1,width=16.5cm}
%%%%%%%%%%
\begin{document}
\ \vskip8cm
\begin{tcolorbox}
\begin{minipage}{8cm}
\begin{enumerate}[font=\sffamily\bfseries,label=\arabic*), series=Is]
\item Si $z=x+iy$ avec $x$ et $y$ réels, alors\\
$|z|=\sqrt{x^2+y^2}$.
\item $z=0\Leftrightarrow |z|=0$
\item $|zz'|=|z|\times |z'|$
\item $\left|z^n\right|=|z|^n$ ($n\in\mathbb{N}^*$)
\end{enumerate}
\end{minipage} \begin{minipage}{8cm}
\begin{enumerate}[label=\arabic*),resume*=Is]
\item $\left|\dfrac{1}{z}\right|=\dfrac{1}{|z|}$ ($z\neq 0$)
\item Si $z'\neq 0$, $\left|\dfrac{z}{z'}\right|=\dfrac{|z|}{|z'|}$
\item Inégalité triangulaire ; $|z+z'|\leqslant |z|+|z'|$ \\
(mais on n'a pas égalité en général) \end{enumerate}
\end{minipage}
\end{tcolorbox}
\end{document}