Vì hệ pt có nghiệm là: (1; -3) nên ta có:
\(\left\{{}\begin{matrix}\left(3a+b\right)\cdot1+\left(4a-b+1\right)\cdot\left(-3\right)=35\\b+4a\cdot\left(-3\right)=29\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}3a+b-12a+3b-3=35\\b-12a=29\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}4b-9a=38\\b-12a=29\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}4b-9a=38\\4b-48a=116\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}39a=-78\\b-12a=29\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{-78}{39}=-2\\b+24=29\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}a=-2\\b=29-24=5\end{matrix}\right.\)
