xy - 4x + y² - 4y = (xy - 4x) + (y² - 4y) = х (y - 4) + у(y - 4) =
(у-4)(х+у)
9a² - 4 = (3а)² - 2² = (3а-2)(3а+2)
a⁶ - 27 = (а²)³ - 3³ = (а² - 3)(а⁴ +3а² + 9)
x³- 4x² + 4x = x(х²- 4x + 4) = х(х-2)² = х(х-2)(х-2)
x³ + x² + 3x + 27 = (x³ + 27) + (x² + 3x) = (x³ + 27) + (x² + 3x) =
= (х+3)(х²-3х+9)+х(х+3) = (х+3)(х²-3х+9+х) = (х+3)(х²-2х+9)