A = 3\(x^2\) + 9\(x\) - 7
A = 3.(\(x^2\) + 3\(x\) + \(\dfrac{9}{4}\)) - 7
A = 3.(\(x\) + \(\dfrac{3}{2}\))2 - \(\dfrac{55}{4}\)
Vì (\(x\) + \(\dfrac{3}{2}\))2 ≥ 0; ⇒ 3.(\(x\) + \(\dfrac{3}{2}\))2 - \(\dfrac{55}{4}\) ≥ - \(\dfrac{55}{4}\)
A(min) = - \(\dfrac{55}{4}\) ⇔ \(x\) + \(\dfrac{3}{2}\) = 0 ⇔ \(x\) = - \(\dfrac{3}{2}\)