y'=5x^4-20x^3+15x^2+20x-20
=5(x^4-4x^3+3x^2+4x-4)
=5[x^2(x^2-4x+3)+4(x-1)]
=5[x^2(x-1)(x-3)+4(x-1)]
=5(x-1)(x^3-3x^2+4)
=5(x-1)[x^2(x-2)-(x^2-4)]
=5(x-1)[x^2(x-2)-(x+2)(x-2)]
=5(x-1)(x-2)(x^2-x-2)
=5(x-1)(x-2)^2(x+1)
当x