\documentclass{exam-zh}
\usepackage{siunitx}
\usepackage{tkz-euclide}
\usetikzlibrary {decorations}
\begin{document}
\begin{tikzpicture}
%\draw[help lines,black,dashed](0,0)grid(5,5);
\tkzDefPoints{0/6/A,6/6/B,6/0/C,0/0/O}
\tkzDefPoints{0/4/A1,4/6/B1,6/4/C1,4/0/O1}
%\tkzDrawPoints[color=black,size=2pt](A,B,C,O)
%\tkzDrawPoints[color=black,size=2pt](A1,B1,C1,O1)
\tkzDrawPolygon(A,B,C,O)
\tkzDrawSegments(A1,C1 B1,O1)
\begin{scope}[/pgf/decoration/raise=2pt]
	\draw [decorate,decoration={brace,mirror,
		amplitude=10pt},xshift=0pt,yshift=-4pt,red](B1) -- (A) node [black,midway,xshift=0pt,yshift=18pt]{\footnotesize $a$};
		\draw [decorate,decoration={brace,mirror,
			amplitude=10pt},xshift=0pt,yshift=-4pt,red](B) -- (B1) node [black,midway,xshift=0pt,yshift=18pt]{\footnotesize $b$};
	\draw [decorate,decoration={brace,mirror,amplitude=10pt},xshift=0pt,yshift=-4pt](A1) -- (O) node [cyan,midway,xshift=-20pt,yshift=0pt]{\footnotesize $a$};
	\draw [decorate,decoration={brace,mirror,amplitude=10pt},xshift=0pt,yshift=-4pt](A) -- (A1) node [cyan,midway,xshift=-20pt,yshift=0pt]{\footnotesize $b$};		
\end{scope}


\end{tikzpicture}

\end{document}