x^3y-xy
=xy(x^2-1)=xy(x-1)(x+1)
-x^3+2x^2-x
= -x(x^2-2x+1)= -x(x-1)^2
2x^2+2y^2-4xy
=2(x^2-2xy+y^2)
=2(x-y)^2
-9x^2+6x-1
=-(3x-1)^2
注：a^b为a的b次方O(∩_∩)O谢谢