用平方差公式：
(3a²+2a-8)²-(a²-2a-8)²
=[(a+2)(3a-4)]²-[(a+2)(a-4)]²
=(a+2)²[(3a-4)²-(a-4)²]
=(a+2)²[(3a-4)+(a-4)][(3a-4)-(a-4)]
=(a+2)²(4a-8)(2a)
=8a(a+2)²(a-2)