a3(b+c)+b3(a+c)+c3(a+b)+abc(a+b+c)
=[a3(b+c)+a2bc]+[b3(a+c)+b2ac]+[c3(a+b)+c2ab]
=a2[ab+ac+bc]+b2[ba+bc+ac]+c2[ca+cb+ab]
=(a2+b2+c2)[ca+cb+ab]