Chứng minh rằng ( x^2+x+1) (x^2-x+1) > 0

`(x^2+x+1)(x^2-x+1) = [(x^2+1)+x][(x^2+1)-x)]`

`= (x^2+1)^2 – x^2 = x^4 + 2x^2 + 1 -x^2  x^4 +x^2+1 > 0 \forall x`

`(x^4 ≥ 0 ; x^2 ≥ 0 <=> x^4+x^2 ≥ 0 <=> x^4+x^2+1≥1 >0)`