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 \numberwithin{equation}{chapter}
\begin{description}
\begin{aligned}[t]
\begin{equation}\label{eq:3}
&\begin{cases}
t = (sz)r' = xr'\\
x = sz = s(yr) = s((zr')r) = (szr')r = tr
\end{cases}\Rightarrow xRt
\end{equation}
\end{aligned}
\end{description}
\begin{description}
\begin{aligned}[t]
\begin{equation}\label{eq:4}
&\begin{cases}
t = s(zr') = sy\\
y = zr' = (s'x)r'= (s'(sz))r'= s'(szr') =s' t
\end{cases} \Rightarrow tLy
\end{equation}
\end{aligned}
\end{description}
از
\eqref{eq:3} 
و
\eqref{eq:4}
نتیجه می‌شود
$ (x,y) \in R.L $.


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