1）
D=|4 1 2 4|
1 2 0 2
2 3 -2 -8 r3-r1*2
0 1 1 7
= - |1 2 0 2| r1与r2交换
4 1 2 4
2 3 -2 -8
0 1 1 7
=-|1 2 0 2|
0 -7 2 -4 r2-r1*4、r3-r1*2
0 -1 -2 -12
0 1 1 7
=|1 2 0 2|
0 1 1 7 r2与r4交换
0 -1 -2 -12
0 -7 2 -4
=|1 2 0 2|
0 1 1 7
0 0 -1 -5 r3+r2、r4+r2*7
0 0 9 45 【其实,这里就可以得出结果了：r4:r3=-9,D=0】
=|1 2 0 2|
0 1 1 7
0 0 -1 -5
0 0 0 0 r4+r3*9
=1*1*(-1)*0=0
2）按列展开降阶
D4=a*|b 1 0| + |1 0 0|
-1 c 1 -1 c 1
0-1 d 0-1 d
=ab|c 1|+ a|1 0| + |c 1| 【第三个二阶行列式是前面按第一行展开】
-1 d -1 d -1 d
=abcd+ab+ad+0+cd+1
=abcd+ab+ad+cd+1