19: Ta có: \(x^2-4y^2+2x-4y\)
\(=\left(x-2y\right)\left(x+2y\right)+2\left(x-2y\right)\)
\(=\left(x-2y\right)\left(x+2y+2\right)\)
20: Ta có: \(x^4-81-6x^3+54x\)
\(=\left(x^2-9\right)\left(x^2+9\right)-6x\left(x^2-9\right)\)
\(=\left(x-3\right)^3\cdot\left(x+3\right)\)
