a: =(x^2-4y^2)-2(x+2y)
=(x+2y)(x-2y)-2(x+2y)
=(x+2y)(x-2y-2)
b: =(x^4-4)+2x^3-4x
=(x^2-2)(x^2+2)+2x(x^2-2)
=(x^2-2)(x^2+2x+2)
c: =x^2(x+2y)-(x+2y)
=(x+2y)(x^2-1)
=(x+2y)(x-1)(x+1)
d: =3(x-y)(x+y)-2(x-y)^2
=(x-y)(3x+3y-2x+2y)
=(x+5y)(x-y)
e: =x^2(x-4)-9(x-4)
=(x-4)(x^2-9)
=(x-4)(x-3)(x+3)
f: =(x-y)(x+y)-2(x+y)
=(x+y)(x-y-2)
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