解: 系数矩阵 =
1 1 -3 -1
3 -1 -3 4
1 5 -9 -8
r2-3r1, r3-r1
1 1 -3 -1
0 -4 6 7
0 4 -6 -7
r3+r2
1 1 -3 -1
0 -4 6 7
0 0 0 0
r2*(-1/4)
1 1 -3 -1
0 1 -3/2 -7/4
0 0 0 0 0
r1-r2
1 0 -3/2 3/4
0 1 -3/2 -7/4
0 0 0 0
通解为: c1(3,3,-2,0)^T + c2(3,-7,0,-4)^T, c1,c2为任意常数.