27) x² - 5x = x(x-5)
28) 3x¹⁰ +9x⁵ = 3x⁵(x⁵ +3)
39) 2x²+x-1 = x² + x² + x - 1 = (x²-1) + (x²+x) = (x² - 1²) + x(x+1) =
= (x-1)(x+1) + x(x+1) = (x+1)(x-1+x) = (x+1)(2x-1).
40) x³+x²-16x-16 = (x³ - 16x) + (x² - 16) = x(x² - 16) + (x² - 16) =
= (x² - 16)(x+1) = (x² - 4²)(x+1) = (x - 4)(x + 4)(x+1).