a: Thay x=2 và y=1 vào hệ, ta được:
\(\left\{{}\begin{matrix}2\cdot a+1\cdot b=2\\2a\cdot2-3b\cdot1=-36\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}2a+b=2\\4a-3b=-36\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}6a+3b=6\\4a-3b=-36\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}10a=-30\\2a+b=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}b=-3\\2a=2-b=2-\left(-3\right)=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}b=-3\\a=\dfrac{5}{2}\end{matrix}\right.\)
b: Thay x=1 và y=2 vào hệ, ta được:
\(\left\{{}\begin{matrix}a\cdot1+b\cdot2=5\\a\cdot1-2\cdot b=3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2a=8\\a-2b=3\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}a=4\\2b=a-3=4-3=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=4\\b=\dfrac{1}{2}\end{matrix}\right.\)