x^4+x^3-4x-16
=（x^2+4)(x^2-4)+x(x^2-4)
=(x^2-4)(x^2+x+4)
x^4+2x^3-9x^2-2x+8
=x^4+2x^3-x^2-2x+8-8x^2
=x^2(x^2-1)+2x(x^2-1)-8(x^2-1)
=(x^-1)(x^2+2x-8)
=(x+1)(x-1)(x+4)(x-2)