\(A=-3x^2+2x-5\)
\(A=-3\left(x^2-\dfrac{2}{3}x+\dfrac{5}{3}\right)\)
\(A=-3\left(x^2-\dfrac{2}{3}x+\dfrac{1}{9}+\dfrac{14}{9}\right)\)
\(A=-3\left(x^2-\dfrac{2}{3}x+\dfrac{1}{9}\right)-\dfrac{14}{3}\)
\(A=-3\left(x-\dfrac{1}{3}\right)^2-\dfrac{14}{3}\le-\dfrac{14}{3}\)
Dấu "=" xảy ra khi: \(x=\dfrac{1}{3}\)
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