\(A=109-\left(x-3\right)^2\le109\)
\(A_{max}=109\) khi \(x=3\)
\(B=x-x^2=\dfrac{1}{4}-\left(x^2-x+\dfrac{1}{4}\right)=\dfrac{1}{4}-\left(x-\dfrac{1}{2}\right)^2\le\dfrac{1}{4}\)
\(B_{max}=\dfrac{1}{4}\) khi \(x=\dfrac{1}{2}\)
\(C=18-4\left(x^2-2x+1\right)-\left(y^2+4y+4\right)=18-4\left(x-1\right)^2-\left(y+2\right)^2\le18\)
\(C_{max}=18\) khi \(\left\{{}\begin{matrix}x=1\\y=-2\end{matrix}\right.\)
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