分解因式:(1)ax﹣ay;(2)x2﹣y4; (3)﹣x2+4xy﹣4y2.
解:(1)ax﹣ay=a(x﹣y);
(2)x2﹣y4=(x+y2)(x﹣y2);
(3)﹣x2+4xy﹣4y2
=﹣(x2﹣4xy+4y2)
=﹣(x﹣2y)2.
“良言一句三冬暖,恶语伤人六月寒。”这句话说明了 ( )