1) xy^4-y^4-xy^2+y^2=y^2(xy^2-y^2-1)
2)x^3+xy+x^3+x=x(x^2+y+x^2+1)=x(2x^2+y+1)
3) a^2b-a^2-ab-a^3=a(ab-a-b-a^2)
4) x^3+xy+x^2y+x^2=x(x^2+y+xy+x)
5)25a^3x^4-10a^2x^2-30ax^6=5ax^2(5a^2x^2-2a-6x^4)
6) 4n(x-2)-3x+6=4n(x-2)-3(x+2)=(4n-3)(x^2-4)
7)64a^2x^2-49b^4x^2=x^2(64a^2-49b^4)
8) 8a^2-5a-8ax+5x=8a(a-x)-5(a+x)=(8a-5)(a^2-x^2)