A = x^2 - 6x + 11
= x^2 - 6x + 9 + 2
= ( x - 3 )^2 +  2 ≥ 2
Dấu =  xảy ra <=> x - 3 = 0 <=> x = 3
A min =  2 khi  x= 3
B = x^2 - 20x + 101
= x^2 - 20x + 100 + 1
= ( x - 10)^2 + 1 ≥ 1
Dấu = xảy ra <=> x - 10 = 0 <=> x = 10
B min = 1 khi x = 10