1、
(x^2+y^2)^2-4x^2y^2
=x^4+2x^2y^2+y^4-4x^2y^2
=x^4-2x^2y^2+y^4
=(x^2-y^2)^2
=(x-y)^2*(x+y)^2
2、
m^2-16m
=m(m-16)
3、
-x^3-x+2x^2
=-x(x^2+1)+2x^2
=-x(x^2+1-2x)
=-x(x-1)^2
4、(-1/3x^2y)(-9xy+1)
=3x^3y^2-1/3x^2y
=1/3（x^2y）（9xy-1)
5、
(x-1)^2-(x-3)(x+1)-(x+1)(x-1)
=x^2-2x+1-(x^2-2x-3)-(x^2-1)
=5-x^2
＝（√5-x）（√5+x)