a, A = 4x² + 4x + 11
= (2x +1)²+10
= (2x +1)² ≥ 0
= (2x +1)² +10 ≥ 10 ∀ x
→ min A = 10 ⇔ 2x+1 = 0 ⇔ x = -1/2
b, B = (x - 1) (x + 2) (x + 3) (x + 6)
= [ (x - 1) (x + 6) ] [ (x + 2) (x + 3) ]
= ( x²+5x-6 ) ( x²+5x+6 )
= ( x²+5x )²- 36
Do ( x²+5x )² ≥ 0
→ ( x²+5x )²- 36 ≥ -36 ∀ x
→ min B = -36 ⇔ x²+5x=0 ⇔ x = 0
x = 5