Ta có
$(x-1)(x+2)(x+3)(x-6)+20x^2$
$= (x-1)(x-6) (x+2)(x+3) + 20x^2$
$= (x^2 -7x + 6)(x^2 + 5x + 6) + 20x^2$
$= [(x^2 + 6) - 7x][(x^2 + 6) + 5x] + 20x^2$
$= (x^2 + 6)^2 -2x(x^2 + 6) -15x^2$
$= x^4 - 2x^3 - 3x^2 - 12x + 36$
$= (x-2)(x^3 -3x -18)$
$= (x-2)(x-3)(x^2 + 3x + 6)$