1.（a-b)²+4a-4b =（a-b)²+4(a-b)=(a-b)(a-b+4)
2.x的n次方+2x的n-1次方-4x的n-2次方 =x^(n-2)(x^2+2x-4)
3.（7a-8b）²-4x²=(7a-8b-2x)(7a-8b+2x)
(4)
a⁴-81
=(a²+9)(a²-9)
=(a²+9)(a+3)(a-3)
(5)
9(a-b)²+12(a-b)+4
=[3(a-b)+2]²
=(3a-3b+2)²
(6)
(a-b)³-4ab(b-a)
=(a-b)³+4ab(a-b)
=(a-b)[(a-b)²+4ab]
=(a-b)(a+b)²