xy^2-4x =x(y+2)(y-2)
-2a^2+4ab-2b^2=-(a-b)^2
a^2(x-y)+b^2(y-x)=(x-y)(a+b)(a-b)
(a^2+b^2)^2-4a^2b^2=(a+b)^2(a-b)^2
81a^4-72a^2b^2+16b^4 =(3a+4b)^2(3a-4b)^2