用立方差公式a³-b³=(a-b)(a²+ab+b²)
(a-1)³-(b+1)³
=[a-1-(b+1)][(a-1)²+(a-1)(b+1)+(b+1)²]
=(a-b-2)(a²-2a+1+ab+a-b-1+b²+2b+1)
=(a-b-2)(a²+ab-a+b²+b+1)