\(D=5x^2-10x-2\)
\(=5\left(x^2-2x+1\right)-7\)
\(=5\left(x-1\right)^2-7\ge-7\)
Vậy \(min_D=-7\)
Để D = -7 thì \(x-1=0\Rightarrow x=1\)
\(E=x^2-2xy+2y^2+y-3\)
\(=\left(x^2-2xy+y^2\right)+\left(y^2+y+\dfrac{1}{4}\right)-\dfrac{13}{4}\)
\(=\left(x-y\right)^2+\left(y+\dfrac{1}{2}\right)^2-\dfrac{13}{4}\ge\dfrac{13}{4}\)
Vậy \(min_E=\dfrac{-13}{4}\)
Để \(E=-\dfrac{13}{4}\) thì \(\left\{{}\begin{matrix}x-y=0\\y+\dfrac{1}{2}=0\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=y=-\dfrac{1}{2}\\y=-\dfrac{1}{2}\end{matrix}\right.\)
