a)\(A=-5x^2-4x+1\)
\(=\frac{9}{5}-\frac{4}{5}-5x^2-4x\)
\(=\frac{9}{5}-\left(5x^2+4x+\frac{4}{5}\right)\)
\(=\frac{9}{5}-5\left(x^2+\frac{4x}{5}+\frac{4}{25}\right)\)
\(=\frac{9}{5}-5\left(x+\frac{2}{5}\right)^2\le\frac{9}{5}\)
Dấu = khi \(-\left(x+\frac{2}{5}\right)^2=0\Leftrightarrow x+\frac{2}{5}=0\Leftrightarrow x=-\frac{2}{5}\)
Vậy \(Max_A=\frac{9}{5}\Leftrightarrow x=-\frac{2}{5}\)
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