a) $A=x^2-6x+11$
$=(x^2-6x+9)+2$
$=(x-3)^3 + 2 ≥ 2$
Dấu "=" xảy ra $⇔x=3$
b) $B=x^2-20x+101$
$=(x^2-20x+100)+1$
$=(x-10)^2+1 ≥1$
Dấu "=" xảy ra $⇔x=10$
c) $C=x^2-4xy+5y^2+10x-22y+28$
$=(x-2y)^2 +2.(x-2y).5 + 25 + y^2-2y+1+2$
$=(x-2y+5)^2+(y-1)^2+2 ≥ 2$
Dấu "=" xảy ra $⇔y=1,x=-3$