a2b+b2c+c2a-ab2-bc2-ca2=a2b-ca2+(b2c-bc2)+(c2a-ab2)=a2(b-c)+bc(b-c)-a(b+c)(b-c)=(b-c)(a2+bc-ab-ac)=(b-c)[(a2-ab)+(bc-ac)]=(b-c)[a(a-b)-c(a-b)]=(b-c)(a-c)(a-b)