a, x2−8y2+6x+9
=(x+3)2−8y2
=(x+3−8y)(x+3+8y)
b, (4x+1)(12x−1)(3x+2)(x+1)−4
=[(4x+1)(3x+2)][(12x−1)(x+1)]−4
=(12x2+8x+3x+2)(12x2+12x−x−1)−4
=(12x2+11x+2)(12x2+11x−1)−4(1)
Đặt 12x2+11x+2=a⇒12x2+11x−1=a−3
⇒(1)=a(a−3)−4=a2−3a−4
=a2+a−4a−4=a(a+1)−4(a+1)
=(a+1)(a−4)(*)
Vì a=12x2+11x+2 nên:
(*)=(12x2+11x+3)(12x2+11x−2)
Chúc bạn học tốt!!!