2(x+y)(x-y)-(x+y)²-(x-y)² 等于多少
问题描述：
2(x+y)(x-y)-(x+y)²-(x-y)² 等于多少
答
2(x+y)(x-y)-(x+y)²-(x-y)²
=-[(x+y)²-2(x+y)(x-y)+(x-y)²]
=-[(x+y)-(x-y)]²
=-(2y)²
=-4y²
运用完全平方公式,要比各项展开再消去简便许多.