\(B=\left[\left(x^2-6xy+9y^2\right)+4\left(x-3y\right)+4\right]+\left(x^2-10x+25\right)+1992\\ B=\left[\left(x-3y\right)^2+4\left(x-3y\right)+4\right]+\left(x-5\right)^2+1992\\ B=\left(x-3y+2\right)^2+\left(x-5\right)^2+1992\ge1992\\ B_{min}=1992\Leftrightarrow\left\{{}\begin{matrix}x=5\\3y=x+2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=5\\y=\dfrac{7}{3}\end{matrix}\right.\)
\(C=\left(-4x^2-4xy-y^2\right)-\left(x^2-x+\dfrac{1}{4}\right)+\dfrac{8089}{4}\\ C=-\left(2x-y\right)^2-\left(x-\dfrac{1}{2}\right)^2+\dfrac{8089}{4}\le\dfrac{8089}{4}\\ C_{max}=\dfrac{8089}{4}\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{1}{2}\\y=2x\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{1}{2}\\y=1\end{matrix}\right.\)
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