r2-r1,r4+5r1
3 1 -1 2
-8 0 4 -6
2 0 1 -1
16 0 -2 9
c1<->c2
D = (-1)*
1 3 -1 2
0 -8 4 -6
0 2 1 -1
0 16 -2 9
r2+4r3,r4-8r3
1 3 -1 2
0 0 8 -10
0 2 1 -1
0 0 -10 17
r2<->r3
D = (-1)*(-1)*
1 3 -1 2
0 2 1 -1
0 0 8 -10
0 0 -10 17
= 1*2*(8*17-10*10)
= 72.
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