2a^2-4ab+2b^2
=2(a^2-2ab+b^2)
=2(a-b)^2
1/4x+x^3-x^2
=x(1/4+x^2-x)
=x(x-1/2)^2
9x^2-y^2-4y-4
=9x^2-(y^2+4y+4)
=9x^2-(y+2)^2
=(3x+y+2)(3x-y-2)
xy^2-2xy+2y-4
=xy(y-2)+2(y-2)
=(xy+2)(y-2)