求数学题答案（至少回答2～3题(3)a²-b²=(a+b)(a-b).(4)(1/3)(a+b+c)²-(1/3)(a²+b²+c²)=(1/3)[(a+b+c)²-(a²+b²+c²)]=(1/3)[(2²+b²+c²+2ab+2ac+2bc)-(a²+b²+c²)]=(1/3)(2ab+2ac+2bc)=(2/3)(ab+ac+bc).
