factorise:
a) 4(x-y)^2-12(x-y)(x+y)+9(x+y)^2
b) (a-b)^3+(b-c)^3+(c-a)^3
a)
if you let a = x-y and b=x+y, you have
4a^2 - 12ab + 9b^2 = (2a-3b)^2
So, you have
(2(x-y)-3(x+y))^2
= (2x-2y-3x+3y)^2
= (y-x)^2
b) you got me. I don't see any simple factors.