4ma^2-16mb^2=4m(a^2-4n^2)=4m(a-2b)(a+2b)
x^3y+xy^3-2x^2y^2=xy(x^2+y^2-2xy)=xy(x-y)^2
a^2b^2-8abc+16c^2=(ab)^2-2(ab)(4c)+(4c)^2=(ab-4c)^2
(a^2+b^2)^2-4a^2b^2=a^4+b^4+2a^2b^2-4a^2b^2=a^4+b^4-2a^2b^2=(a^2-b^2)^2=(a-b)^2(a+b)^2