a)A= x² + 6x + 15 = (x² + 6x + 9 ) + 6 = (x+3)² + 6
ta có (x+3)²≥ 0 ⇒ (x+3)² + 6 ≥ 6
⇒ MinA = 6 ⇔ x + 3 = 0 ⇔ x= -3
b)A= x² - 17 + 4y² + 8x +4y
⇒ (x² +8x + 16) + (4y² +4y + 1) - 34
⇒(x +4)² + (2y +1)² - 34
Vì (x +4)²≥0; (2y +1)² ≥ 0
⇒(x +4)² + (2y +1)² - 34≥ -34
⇒MinA= -34⇔ $\left \{ {{x+4=0} \atop {2y+1=0}} \right.$ ⇔$\left \{ {{x=-4} \atop {y = \frac{-1}{2}}} \right.$
