a的3n次方-4a的2n次方-16a的n次方+64
=a的2n次方×(a的n次方-4)-16×(a的n次方-4)
=(a的n次方-4)(a的2n次方-16)
=(a的n次方-4)(a的n次方-4)(a的n次方+4)
=(a的n次方-4)²(a的n次方+4)