(4) (a+b+c)(ab+bc+ca)-abc ={a+(b+c)}{(b+c)a+bc}-bca =(b+c)a²+bca+(b+c)²a+bc(b+c)-bca =(b+c)a²+(b+c)²a+bc(b+c) =(b+c){a²+(b+c)a+bc} =(b+c)(a+b)(a+c)=(a+b)(b+c)(c+a) 21