\(A=-\left(x^2+8xy+16y^2\right)-\left(2x^2-5x\right)+2\\ A=-\left(x+4y\right)^2-2\left(x^2-2\cdot\dfrac{5}{4}x+\dfrac{25}{16}\right)+\dfrac{41}{8}\\ A=-\left(x+4y\right)^2-2\left(x-\dfrac{5}{4}\right)^2+\dfrac{41}{8}\le\dfrac{41}{8}\)
Dấu \("="\Leftrightarrow\left\{{}\begin{matrix}x+4y=0\\x=\dfrac{5}{4}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{5}{4}\\y=-\dfrac{x}{4}=-\dfrac{5}{16}\end{matrix}\right.\)
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