天堂无泪
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(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+2ab+ac)+(2ab+4b²+2bc)+(ac+2ab+c²)]
=(a²-2ab-ac-2ab+4b²+2bc-ac+2ab+c²)-(a²+2ab+ac+2ab+4b²+2bc+ac+2ab+c²)
=(a²-4ab-2ac+2bc+4b²+c²)-(a²+4ab+2ac+2bc+4b²+c²)
=-8ab-4ac
不懂可以问我^^
1年前
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天堂无泪
[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