A=x2−2x+50
A=x2−2x+1+49
A=(x−1)2+49≥49
Dấu "=" xảy ra khi:
x=1
B=12x−x2
B=−x2+12x
B=−x2+12x−36+36
B=−(x2−12x+36)+36
B=−(x−6)2+36≤36
Dấu "=" xảy ra khi:
x=6
C=(x+1)(x−2)(x−3)(x−6)
C=[(x+1)(x−6)][(x−2)(x−3)]
C=[x(x−6)+1(x−6)][x(x−3)−2(x−3)]
C=(x2−6x+x−6)(x2−3x−2x+6)
C=(x2−5x−6)(x2−5x+6)
C=(x2−5x)2−36≥−36
Dấu "=" xảy ra khi:
x2−5x=0
⇒x(x−5)=0
⇒[x=0x=5