\(=x^4+1999x^2-x+1999x+1999\)
\(=\left(x^4-x\right)+\left(1999x^2+1999x+1999\right)\)
\(=x\left(x^3-1\right)+1999\left(x^2+x+1\right)\)
\(=x\left(x-1\right)\left(x^2+x+1\right)+1999\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^2-x+1999\right)\)