Question

Tìm GTNN của 

\(A=4x^2-4x-7\)

Answer 1

\(A=4x^2-4x-7=\left(4x^2-4x+1\right)-8=\left(2x-1\right)^2-8\ge-8\)

\(minA=-8\Leftrightarrow x=\dfrac{1}{2}\)

Answer 2

\(\Leftrightarrow A=\left(4x^2-4x+1\right)-8=\left(2x-1\right)^2-8\ge-8\)

Dấu "=" xảy ra \(\Leftrightarrow x=\dfrac{1}{2}\)

Vậy \(A_{min}=-8\Leftrightarrow x=\dfrac{1}{2}\)