\documentclass[12]{article}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage{amsthm}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage[top=3cm, bottom=3cm, left=3cm, right=3cm]{geometry}
\usepackage[x11names]{xcolor}
\usepackage{framed, etoolbox}
\colorlet{framecolor}{VioletRed4}
\colorlet{shadecolor}{LavenderBlush2!60}
\usepackage{thmtools} %
\makeatletter
\define@key{thmdef}{frame}[{}]{%
\thmt@trytwice{}{%
\RequirePackage{framed}%
\RequirePackage{thm-patch}%
\def\FrameCommand{\fcolorbox{framecolor}{shadecolor}}
\addtotheorempreheadhook[\thmt@envname]{%
\begin{framed}}%
\addtotheorempostfoothook[\thmt@envname]{\end{framed}}%
}%
}
\makeatother
\declaretheorem[numberwithin=section, frame]{definition}
\declaretheorem[frame, sharenumber=definition]{proposition}
\AtBeginEnvironment{proposition}{\colorlet{framecolor}{DarkSeaGreen4}
\colorlet{shadecolor}{Yellow1!15}}
\newcommand{\myarrow}[1][1cm]{\mathrel{%
\hbox{\rule[\dimexpr\fontdimen22\textfont2-.2pt\relax]{#1}{.4pt}}%
\mkern-4mu\hbox{\usefont{U}{lasy}{m}{n}\symbol{41}}}}
\makeatletter
\setbox0\hbox{$\xdef\scriptratio{\strip@pt\dimexpr
\numexpr(\sf@size*65536)/\f@size sp}$}
\newcommand{\myscriptarrow}[1][1cm]{{%
\hbox{\rule[\scriptratio\dimexpr\fontdimen22\textfont2-.2pt\relax]
{\scriptratio\dimexpr#1\relax}{\scriptratio\dimexpr.4pt\relax}}%
\mkern-4mu\hbox{\let\f@size\sf@size\usefont{U}{lasy}{m}{n}\symbol{41}}}}
\makeatother
\begin{document}
\section{Arithmérique}
\subsection{12}
le but de ce chapitre
\begin{definition}
Soit f:$G \rightarrow G$ une application linéaire alors:\\
$$\forall x,y\in G,\ f(x)=f(y)\implies x=y$$
\end{definition}
\begin{proposition}
Soit $f:G \myarrow[.7cm]G$ une application linéaire alors:\\
$$\forall x,y\in G,\ f(x)=f(y)\iff x=y$$
\end{proposition}
\end{document}
答案1
这是怎么回事？你可以将你的代码与我输入的代码进行比较，以了解答案。抱歉我写信的速度太快了。
附录：非常感谢@Mico 的建议（见评论）。
\documentclass[12pt]{article}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage{amsthm}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage[utf8]{inputenc}
\usepackage[top=3cm, bottom=3cm, left=3cm, right=3cm]{geometry}
\usepackage[x11names]{xcolor}
\usepackage{framed, etoolbox}
\colorlet{framecolor}{VioletRed4}
\colorlet{shadecolor}{LavenderBlush2!60}
\usepackage{thmtools} %
\usepackage{titlesec}%%%%%% <---------addendum
\titleformat{\section}%%%%%% <---------addendum
{\color{magenta}\normalfont\Large\bfseries}%%%%%% <---------addendum
{\color{magenta}\thesection}{1.5em}{}%%%%%% <---------addendum
\makeatletter
\define@key{thmdef}{frame}[{}]{%
\thmt@trytwice{}{%
\RequirePackage{framed}%
\RequirePackage{thm-patch}%
\def\FrameCommand{\fcolorbox{framecolor}{shadecolor}}
\addtotheorempreheadhook[\thmt@envname]{%
\begin{framed}}%
\addtotheorempostfoothook[\thmt@envname]{\end{framed}}%
}%
}
\makeatother
\declaretheorem[numberwithin=section, frame]{definition}
\declaretheorem[frame, sharenumber=definition]{proposition}
\AtBeginEnvironment{proposition}{\colorlet{framecolor}{DarkSeaGreen4}
\colorlet{shadecolor}{Yellow1!15}}
\newcommand{\myarrow}[1][1cm]{\mathrel{%
\hbox{\rule[\dimexpr\fontdimen22\textfont2-.2pt\relax]{#1}{.4pt}}%
\mkern-4mu\hbox{\usefont{U}{lasy}{m}{n}\symbol{41}}}}
\makeatletter
\setbox0\hbox{$\xdef\scriptratio{\strip@pt\dimexpr
\numexpr(\sf@size*65536)/\f@size sp}$}
\newcommand{\myscriptarrow}[1][1cm]{{%
\hbox{\rule[\scriptratio\dimexpr\fontdimen22\textfont2-.2pt\relax]
{\scriptratio\dimexpr#1\relax}{\scriptratio\dimexpr.4pt\relax}}%
\mkern-4mu\hbox{\let\f@size\sf@size\usefont{U}{lasy}{m}{n}\symbol{41}}}}
\makeatother
\begin{document}
\section{Arithmérique}
\subsection{12}
le but de ce chapitre
\begin{definition}
Soit $f\colon G \rightarrow G$ une application linéaire alors:\\%%%%%% <---------edit
\[\forall x,y\in G,\ f(x)=f(y)\implies x=y\]
\end{definition}
\begin{proposition}
Soit $f\colon G \myarrow[.7cm]G$ une application linéaire alors:\\%%%%%% <---------edit
\[\forall x,y\in G,\ f(x)=f(y)\iff x=y\]
\end{proposition}
\end{document}
附录（2）：还有另一种可能性：使用@Bernard 的评论。
\documentclass[12pt]{article}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage{amsthm}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage[utf8]{inputenc}
\usepackage[top=3cm, bottom=3cm, left=3cm, right=3cm]{geometry}
\usepackage[x11names]{xcolor}
\usepackage{framed, etoolbox}
\colorlet{framecolor}{VioletRed4}
\colorlet{shadecolor}{LavenderBlush2!60}
\usepackage{thmtools} %
\usepackage{titlesec}%%%%%% <---------addendum
\titleformat*{\section}{\color{magenta}\Large\bfseries} %%%%%% <---------@Bernard's comment
\makeatletter
\define@key{thmdef}{frame}[{}]{%
\thmt@trytwice{}{%
\RequirePackage{framed}%
\RequirePackage{thm-patch}%
\def\FrameCommand{\fcolorbox{framecolor}{shadecolor}}
\addtotheorempreheadhook[\thmt@envname]{%
\begin{framed}}%
\addtotheorempostfoothook[\thmt@envname]{\end{framed}}%
}%
}
\makeatother
\declaretheorem[numberwithin=section, frame]{definition}
\declaretheorem[frame, sharenumber=definition]{proposition}
\AtBeginEnvironment{proposition}{\colorlet{framecolor}{DarkSeaGreen4}
\colorlet{shadecolor}{Yellow1!15}}
\newcommand{\myarrow}[1][1cm]{\mathrel{%
\hbox{\rule[\dimexpr\fontdimen22\textfont2-.2pt\relax]{#1}{.4pt}}%
\mkern-4mu\hbox{\usefont{U}{lasy}{m}{n}\symbol{41}}}}
\makeatletter
\setbox0\hbox{$\xdef\scriptratio{\strip@pt\dimexpr
\numexpr(\sf@size*65536)/\f@size sp}$}
\newcommand{\myscriptarrow}[1][1cm]{{%
\hbox{\rule[\scriptratio\dimexpr\fontdimen22\textfont2-.2pt\relax]
{\scriptratio\dimexpr#1\relax}{\scriptratio\dimexpr.4pt\relax}}%
\mkern-4mu\hbox{\let\f@size\sf@size\usefont{U}{lasy}{m}{n}\symbol{41}}}}
\makeatother
\begin{document}
\section{Arithmérique}
\subsection{12}
le but de ce chapitre
\begin{definition}
Soit $f\colon G \rightarrow G$ une application linéaire alors:\\%%%%%% <---------edit
\[\forall x,y\in G,\ f(x)=f(y)\implies x=y\]
\end{definition}
\begin{proposition}
Soit $f\colon G \myarrow[.7cm]G$ une application linéaire alors:\\%%%%%% <---------edit
\[\forall x,y\in G,\ f(x)=f(y)\iff x=y\]
\end{proposition}
\end{document}