1: \(x^{16}-81=\left(x^8-9\right)\left(x^8+9\right)=\left(x^4-3\right)\left(x^4+3\right)\left(x^8+9\right)\)
2: \(x^6-9x^4+27x^2-27=\left(x^2-3\right)^3\)
8: \(8x^3-12x^2+6x-1=\left(2x-1\right)^3\)
4: \(x^2+3xy+3x+9y\)
\(=x\left(x+3y\right)+3\left(x+3y\right)\)
\(=\left(x+3y\right)\left(x+3\right)\)