Q=2x²−6x
Q=2(x²- 3x)
Q = 2 ( x ² − 2. 3/2 . x + 9/4 ) − 9/ 2
Q=2.(x²− 2 .3/2 . x + 9/4)−9/2
Q = 2. ( x − 3 /2 )² − 9/2 ≥ − 9/ 2
Q=2.(x−3/2)²−9/2 ≥−9/2
⇒MinQ=−9/2 ⇔ 2(x-3/2)=0 ⇔ x-3/2=0 ⇔ x=3/2
vậy MinQ = -9/2 khi x=3/2
C= 2x²+6x
C=2(x²+3x)
C=2(x²+2.x.3/2 + 9/4) -9/4
C=2(x+3/4)² -9/4 ≤ -9/4
=> MaxC= -9/4 ⇔2(x+3/2)=0 ⇔ x+3/2=0 ⇔ x=-3/2
vậy MaxC = -9/2 khi x=-3/2