r2+2r1,r4+r1,r1*(1/2) [第1行提出2],r3+3r1
-1 1 -2 0
0 3 -5 5
0 4 -8 -3
0 2 1 1
r2-r4,r3-2r4
-1 1 -2 0
0 1 -6 4
0 0 -10 -5
0 2 1 1
r4-2r2
-1 1 -2 0
0 1 -6 4
0 0 -10 -5
0 0 13 -7
= 2*(-1)*1*(10*7+5*13)
= -2*135
= -270.