e: \(\left\{{}\begin{matrix}\dfrac{-b}{2a}=8\\\dfrac{-\left(b^2-4ac\right)}{4a}=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-b=16a\\-b^2+4ac=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}b=-16a\\-256a^2+4ac=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}b=-16a\\-4a\left(64a-c\right)=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}b=-16a\\c=64a\end{matrix}\right.\)
Thay x=6 và y=-12 vào (P), ta được:
\(36a+6b+c=-12\)
\(\Leftrightarrow36a-96a+64a=-12\)
\(\Leftrightarrow a=-3\)
\(\Leftrightarrow\left\{{}\begin{matrix}b=-48\\c=192\end{matrix}\right.\)
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