x^3-x^2-5x-3=x^3+1-(x^2+5x+4)
=(x+1)(x^2-x+1)-(x+1)(x+4)
=(x+1)(x^2-2x-3)
=(x-3)(x+1)^2
(x-3)(x+1)^2/(x+1)=(x-3)(x+1)
x^3-x^2-5x-3是被x+1整除