答 (1) a(b+c)²+b(c+a)²+c(a+b)²-4abc NO =a(b+c)²+b(c²+2ca+a²)+c(a²+2ab+b²)-4abc =(b+c)a²+{(b+c)²+2bc+2bc-4bc}a+bc²+b²c =(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)