\(A=12x-8y-4x^2-y^2+1\)
\(=\left(-4x^2+12x-9\right)+\left(-y^2-8y-16\right)+26\)
\(=-\left(4x^2-12x+9\right)-\left(y^2+8y+16\right)+26\)
\(=-\left(2x-3\right)^2-\left(y+4\right)^2+26\)
ta có: \(\left\{{}\begin{matrix}-\left(2x-3\right)^2\le0\\-\left(y+4\right)^2\le0\end{matrix}\right.\Rightarrow A=-\left(2x-3\right)^2-\left(y+4\right)^2+26\le0+0+26=26\)
Dấu "=" xảy ra khi \(\left\{{}\begin{matrix}x=\dfrac{3}{2}\\y=-4\end{matrix}\right.\)
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