we will do it again from beginning
we have
(x2+3x+3)1/3>0 ∀x∈ℝ
(2x2+3x+2)1/3>0 ∀x∈ℝ
and 6x2+12x+8>0 ∀x∈ℝ
then
apply cauchy inequality (A+A1+A2+...+An)/n ≥(A*A1*A2*...*An)1/n; (A,A1,A2,...,An≥0)
6x2+12x+8=(1*1(x2+3x+3))1/3+(1*1(2x2+3x+2))1/3≤(x2+3x+3+1+1)/3+(2x2+3x+2+1+1)/3=x2+2x+3...