a: \(A=2x^2-8x+1\)
\(=2\left(x^2-4x+\dfrac{1}{2}\right)\)
\(=2\left(x^2-4x+4-\dfrac{7}{2}\right)\)
\(=2\left(x-2\right)^2-7>=-7\forall x\)
Dấu '=' xảy ra khi x-2=0
=>x=2
b: \(B=4x^2-12x+1\)
\(=4x^2-12x+9-8\)
\(=\left(2x-3\right)^2-8>=-8\forall x\)
Dấu '=' xảy ra khi 2x-3=0
=>2x=3
=>\(x=\dfrac{3}{2}\)
c: \(I=12x^2+18x-4\)
\(=12\left(x^2+\dfrac{3}{2}x-\dfrac{1}{3}\right)\)
\(=12\left(x^2+2\cdot x\cdot\dfrac{3}{4}+\dfrac{9}{16}-\dfrac{9}{16}-\dfrac{1}{3}\right)\)
\(=12\left[\left(x+\dfrac{3}{4}\right)^2-\dfrac{43}{48}\right]\)
\(=12\left(x+\dfrac{3}{4}\right)^2-\dfrac{43}{4}>=-\dfrac{43}{4}\forall x\)
Dấu '=' xảy ra khi \(x+\dfrac{3}{4}=0\)
=>\(x=-\dfrac{3}{4}\)
