a, M = 4x2 - 2x + \(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)
M = (2x - \(\dfrac{1}{2}\))2 + \(\dfrac{3}{4}\)
Vì (2x - \(\dfrac{1}{2}\))2 ≥ 0 ∀ x
⇒ (2x - \(\dfrac{1}{2}\))2 + \(\dfrac{3}{4}\) ≥ \(\dfrac{3}{4}\) ∀ x
⇔ M ≥ \(\dfrac{3}{4}\) ∀ x
⇔ Mmin = \(\dfrac{3}{4}\)