分解y^4-4y^3-2y^2+9y-4（厉害的来）

y^4-4y^3-2y^2+9y-4
=y^4-y^3-(3y^3-3y^2)-(5y^2-5y)+(4y-4)
=y^3(y-1)-3y^2(y-1)-5y(y-1)+4(y-1)
=(y-1)(y^3-3y^2-5y+4)
=(y-1)(y^3-4y^2+y^2-4y-y+4)
=(y-1)[y^2(y-4)+y(y-4)-(y-4)]
=(y-1)(y-4)(y^2+y-1)