\(\left(x^2+x+1\right)\left(x^5-x^4+x^2-x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^5-x^4\right)+\left(x^2+x+1\right)\left(x^2-x+1\right)\)
\(=x^4\left(x-1\right)\left(x^2+x+1\right)+\left(x^2+1\right)^2-x^2\)
\(=x^4\left(x^3-1\right)+x^4+2x^2+1-x^2\)
\(=x^7-x^4+x^4+x^2+1\)
\(=x^7+x^2+1\)