1/3x^2y-3y=1/3y(x^2-9)=y/3(x+3)(x-3)
,(y^2＋4)^2-16y^2=(y^2+4+4y)(y^2+4-4y)=(y+2)^2(y-2)^2=(y^2-4)^2
,(a^2＋b^2)^2-4a^2b^2=(a^2+b^2-2ab)(a^2+b^2+2ab)
=(a-b)^2(a+b)^2=(a^2-b^2)^2