x^3-9x =x(x^2-9)=x(x+3)(x-3)16X^4-1=(4x^2+1)(2x+1)(2x-1)6xy^2 -9x^2y-y^3=-y(9x^2-6xy+y^2)=-y(3x-y)^2
题目写清楚一点
