(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