(b^2-c^2)a+(c^2-a^2)b+(a^2-b^2)c
=-(b-c)a^2 + (b+c)(b-c)a - bc(b-c)
=-(b-c){a^2 - (b+c)a + bc}
=-(b-c)(a-b)(a-c)
=(a-b)(b-c)(c-a)