(1)a4-1; 原式=(a2+1)(a2-1) =(a2+1)(a+1)(a-1); (2)x3-2x2y+xy2. 原式=x(x2-2xy+y2) =x(x-y)2. (3)(a-b)(x-y)-(b-a)(x+y); 原式=(a-b)(x-y)+(a-b)(x+y) =(a-b)[(x-y)+(x+y)] =2x(a-b), (4)(x2+1)2-4x2. 原式=(x2+1)2-(2x)2 =(x2+1+2x)(x2+1-2x) =(x+1)2(x-1)2.