因式分解（1）a2(x?y)+b2(y?x)（2）x4?18x2+81

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（1）(x ?y) (a + b) (a ?b)（2）(x + 3)2 (x? 3)2解析(1) 解：原式= a2(x?y)?b2(x ?y) =(x ?y) ( a2 ?b2) =(x ?y) (a + b) (a ?b)
(2) 解：原式= (x2? 9)2 = [(x + 3) (x? 3)]2 = (x + 3)2 (x? 3)2