r1-2r2,r3+r2
0 4 -5 -4
1 0 3 2
0 5 0 3
0 1 0 2
r1-4r4,r3-5r4
0 0 -5 4
1 0 3 2
0 0 0 -7
0 1 0 2
秩为4