\(A=2x^2+10x-1=2\left(x^2+5x-\dfrac{1}{5}\right)=2\left(x^2+5x+\dfrac{25}{4}-\dfrac{129}{20}\right)=2\left(x^2+5x+\dfrac{25}{4}\right)-\dfrac{129}{10}=2\left(x+\dfrac{5}{2}\right)^2-\dfrac{129}{10}\ge-\dfrac{129}{10}\)
Dấu "=" xảy ra khi: \(x=-\dfrac{5}{2}\)
