Đáp án:Q=x^2 - 5x
=( x² - 5x + $\frac{25}{4}$) - $\frac{25}{4}$
=(x - $\frac{5}{2}$ )² - $\frac{25}{4}$
Vì (x - $\frac{5}{2}$ )² $\geq$ 0 ∀ x => (x - $\frac{5}{2}$ )² - $\frac{25}{4}$ ≥ - $\frac{25}{4}$ ∀x
Dấu "=" xảy ra <=> x = $\frac{5}{2}$
Vậy MinQ =- $\frac{25}{4}$ <=> x = $\frac{5}{2}$.