线性代数 D=7 -3 2 5 4 7 -1 -3 2 -5 1 2 2 -6 1 4 怎么算

7 -3 2 5
4 7 -1 -3
2 -5 1 2
2 -6 1 4
r1-2r3,r2+r3,r4-r3
3 7 0 1
6 2 0 -1
2 -5 1 2
0 -1 0 2
按第3列展开
3 7 1
6 2 -1
0 -1 2
c3+2c2
3 7 15
6 2 3
0 -1 0
r2-2r1
3 7 15
0 -12 -27
0 -1 0
= 3*27 = 81