a4＋a3－3a2－4a－4
=a²(a²+a-2)-(a²+4a+4）
=a²(a+2)(a-1)-(a+2)²
=(a+2)(a³-a²-a-2)
=(a+2)(a³-a²-2a+a-2)
=(a+2)[a(a-2)(a+1)+(a-2)]
=(a+2)(a-2)(a²+a+1）