tìm giá trị nhỏ nhất A= 9x^2-6x-4

\(\begin{array}{l}
A = 9{x^2} – 6x – 4 = {\left( {3x} \right)^2} – 2.3x.1 + 1 – 5\\
= {\left( {3x – 1} \right)^2} – 5\\
{\left( {3x – 1} \right)^2} \ge 0 \Rightarrow {\left( {3x – 1} \right)^2} – 5 \ge – 5 \Leftrightarrow A \ge – 5\\
\Rightarrow {A_{\min }} = – 5 \Leftrightarrow 3x – 1 = 0 \Leftrightarrow x = \frac{1}{3}
\end{array}\)