就按第1列展开即可
1 a 0 0
0 1 a 0
0 0 1 a
a 0 0 1
=
1 a 0 + (-a) * a 0 0
0 1 a 1 a 0
0 0 1 0 1 a
显然
1 a 0 =1
0 1 a
0 0 1
而
a 0 0 =a^3
1 a 0
0 1 a
所以行列式的值为1-a*a^3=1 -a^4