Đáp án:
A = (a-b)(b-c)(a-c)+(a+b)(c+a)(c-b)+(b+c)(c+a)(b-a)
= ( b - c) [ a2−ab−ac+bc−ac−bc−a2−ab]+(b+c)(c+a)(b−a)a2−ab−ac+bc−ac−bc−a2−ab]+(b+c)(c+a)(b−a)
= -2a.(b + c) (b - c) + (b+c)(c+a)(b-a)
= ( b + c ) ( bc + ab - ac - a2a2 - 2ab + 2ac )
= ( b + c ) ( bc - ab + ac - a2a2 )
= ( b + c ) ( a + b ) ( c - a )