\(A=-3\left(x^2-4x+4\right)+12=-3\left(x-2\right)^2+12\le12\)
\(A_{max}=12\) khi \(x=4\)
\(B=-\left(x^2-2x+1\right)-\left(x^2+2xy+y^2\right)+1=-\left(x-1\right)^2-\left(x+y\right)^2+1\le1\)
\(B_{max}=1\) khi \(\left(x;y\right)=\left(1;-1\right)\)
\(C=-\left(4x^2+12x+9\right)+9=-\left(2x+3\right)^2+9\le9\)
\(C_{max}=9\) khi \(x=-\dfrac{3}{2}\)
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A = - 3 (x^2 - 4x + 4) + 12
= - 3 (x - 2)^2 + 12 ≤ 12
= max => 12 khi x = 4
B = - (x^2 - 2x + 1) - (x^2 + 2xy + y^2) + 1
= - (x - 1)^2 - (x + y ) + 1 ≤ 1
= max => 1 khi (x,y) khi (1,1)
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