(x-y)^3+ (y-z)^3+ (z-x)^3=x^3 –3(x^2)*y + 3xy^2 -
y^3 + y^3 –3(y^2)*z + 3yz^2 - z^3 + z^3 –3(z^2)*x + 3zx^2 -
x^3=3*(-(x^2)*y + xy^2– (y^2)*z + yz^2–(z^2)*x + zx^2)
-(x^2)*y + xy^2– (y^2)*z + yz^2–(z^2)*x + zx^2=-xy(x-y) +z(x^2 – y^2)
–(z^2)(x-y)=(x-y)(-xy+zx+zy-z^2)=
(x-y)((-x)(y-z)+z(y-z))=(x-y)(y-z)(z-x)
