\(A=4x^2+4x+8\)
\(=4\left(x^2+x+\dfrac{1}{4}\right)+7\)
\(=4\left(x+\dfrac{1}{2}\right)^2+7\ge7\forall x\)
Vậy Min A = 7 khi \(x+\dfrac{1}{2}=0\Rightarrow x=-\dfrac{1}{2}\)
\(B=\left(2x-1\right)^2+\left(x+2\right)^2\)
\(=4x^2-4x+1+x^2+4x+4\)
\(=5x^2+5\)
Vậy Min B = 5 khi \(x=0\)
\(C=x^2+10x+26+y^2+2y+2020\)
\(=\left(x^2+10x+25\right)+\left(y^2+2y+1\right)+2020\)
\(=\left(x+5\right)^2+\left(y+1\right)^2+2020\ge2020\forall x\)
Vậy Min C = 2020 khi \(\left\{{}\begin{matrix}x+5=0\\y+1=0\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=-5\\y=-1\end{matrix}\right.\)
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