x^4-2x^2y-3y^2+8y-4=x^4-2x^2y+y^2-4y^2+8y-4=(x^4-2x^2y+y^2)-(4y^2-8y+4)=(x^2-y)^2-(2y-2)^2 平方差公式=(x^2-y+2y-2)(x^2-y-2y+2)=(x^2+y-2)(x^2-3y+2)