解: 第1行提出 a-1
10 -1
-1a -1
0 -1 a-1
r2+r1
10 -1
0a -2
0 -1 a-1
D = (a-1)[a(a-1)-2]
= (a-1)[a^2-a-2]
= (a-1)(a-2)(a+1)