r2-3r1,r5-5r1
1 1 1 1 1
0 -1 -2 -2 -6
0 1 2 2 6
0 -1 13 13 29
r3+r2,r3-r2
1 1 1 1 1
0 -1 -2 -2 -6
0 0 0 0 0
0 0 15 15 35
矩阵的秩 = 3.