Given: f(x)=−2x3−9x2−12x+1 Differentiating, both sides, we get ⇒f’(x)=−6x2−18x−12 For f(x) to be decreasing function, ∴f’(x)<0 ⇒−6x2−18x−12<0 ⇒−6(x2+3x+2)<0 Multiplying each side of an inequality by a negative number reverses the direction of the inequality symbol ⇒(x2+3x+2)>0 ⇒(x+2)(x+1)>0