1) \(D=-\left(2x-5\right)^2-4\left(2x-5\right)+5\)
\(=-\left[\left(2x-5\right)^2+4\left(2x-5\right)+4\right]+9\)
\(=-\left(2x-5+2\right)^2+9\)
\(=-\left(2x-3\right)^2+9\le9\)
\(maxD=9\Leftrightarrow x=\dfrac{3}{2}\)
2) \(D=x^2-4xy+5y^2+10x-22y+28\)
\(=\left[x^2-2x\left(2y-5\right)+\left(2y-5\right)^2\right]+\left(y^2-2y+1\right)+2\)
\(=\left(x-2y+5\right)^2+\left(y-1\right)^2+2\ge2\)
\(minD=2\Leftrightarrow\) \(\left\{{}\begin{matrix}x=7\\y=1\end{matrix}\right.\)
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