(d3)//(d4)\(\Leftrightarrow\left\{{}\begin{matrix}m^2+6m=7\\2n+7\ne-n^2-9\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}m^2+6m-7=0\\n^2+2n+16\ne0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\left(m-1\right)\left(m+7\right)=0\\\left(n+1\right)^2+15\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}m-1=0\\m+7=0\end{matrix}\right.\\\left(n+1\right)^2+15\ne0\left(luônđúng\right)\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}m=1\\m=-7\end{matrix}\right.\)
\(\left(d3\right)\equiv\left(d4\right)\Leftrightarrow\left\{{}\begin{matrix}m^2+6m=7\\2n+7=-n^2-9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}m^2+6m-7=0\\n^2+2n+16=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\left(m-1\right)\left(m+7\right)=0\\\left(n+1\right)^2+15=0\left(vôlí\right)\end{matrix}\right.\)
