Đáp án:
$A = x² + 2x + 5 $
$A = x² + 2x + 1 + 4 $
$A = (x + 1)² + 4 \geq 4 ∀ x $
$⇒ Amin = 4 ⇔ (x + 1)² = 0 $
$ ⇔ x = - 1 $
$B = x² - x - 51 $
$B = x² - 2.\frac{1}{2}x + \frac{1}{4} - \frac{205}{4} $
$B = (x - \frac{1}{2})² - \frac{205}{4} \geq - \frac{205}{4}$
$⇒ Bmin = - \frac{205}{4} ⇔ x = \frac{1}{2} $