答案：(x²-y²)²
设 x²为a,y²为b
原式= a²+b²+2ab-4ab
= a²+b²-2ab
= (a-b)²
将 x²,y²代入,
原式=(a-b)²= (x²-y²)²=【 (x+y)(x-y)】²= (x+y)²(x-y)²