\(A=x^2-2x.\frac{3}{2}+\frac{9}{4}+\frac{11}{4}\)
\(A=\left(x-\frac{3}{2}\right)^2+\frac{11}{4}\ge\frac{11}{4}\)
MIN A=\(\frac{11}{4}\Leftrightarrow x=\frac{3}{2}\)
Ta có : A = x2 - 6x + 15
= x2 - 6x + 9 + 6
= (x - 3)2 + 6 ≥6∀x∈R
Vậy Amin = 6 khi x = 3.