27. x² - 5x = x(x - 5)
28. 3x - 9x⁸ =
29. x(x + 1) - x - 1 = x(x + 1 ) - (x + 1) =
(x + 1)(x - 1) = (x² - 1)
30. x²(x + 1) + x(x + 1)² = x[x(x + 1) + (x + 1)²] =
x[(x + 1)(2x + 1)]
31. xy - x - y + 1 = y(x - 1) - (x - 1) = (x - 1)(y - 1)
32. 9xy + 3x - 3y - 1 = 3x(3y + 1) - (3y + 1) =
(3y + 1)(3x - 1)
33. 25x² - 49 = (5x + 7)(5x - 7)
