(a-2b-c)²-(a+2b+c)²=(a-2b-c)(a-2b-c)-(a+2b+c)(a+2b+c) =[a(a-2b-c)-2b(a-2b-c)-c(a-2b-c)]-[a(a+2b+c)+2b(a+2b+c)+c(a+2b+c)] =[(a²-2ab-ac)-(2ab-4b²-2bc)-(ac-2ab-c²)]-[(a²...用[a-(2b+c)]²-[a+(2b+c)]²答[a-(2b+c)]²-[a+(2b+c)]²设2b+c为y，得 (a-y)²-(a+y)²=(a²-2ay+y²)-(a²+2ay+y²)=-4ay把y=2b+c代入，得=-4a(2b+c)=-8ab-4ac恩，这种方法简便多了