\(A=x^2+2y^2-2xy-4y+2016\)
\(=\left(x^2-2xy+y^2\right)+\left(y^2-4y+4\right)+2012\)
\(=\left(x-y\right)^2+\left(y-2\right)^2+2012\)\(\ge\)\(2012\), \(\forall x,y\)
Dấu "=" xảy ra \(\Leftrightarrow\)\(\hept{\begin{cases}x-y=0\\y-2=0\end{cases}}\)\(\Leftrightarrow\)\(\hept{\begin{cases}x=y=2\\y=2\end{cases}}\)
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