(a1,a2,a3,a4)=
1 2 1 3
-1 1 -1 0
0 5 -2 7
4 6 0 14
r2+r1,r4-4r1
1 2 1 3
0 3 0 3
0 5 -2 7
0 -2 -4 2
r2*(1/3),r1-2r2,r3-5r2,r4+2r2
1 0 1 1
0 1 0 1
0 0 -2 2
0 0 -4 4
r3*(-1/2),r1-r3,r4+4r3
1 0 0 2
0 1 0 1
0 0 1 -1
0 0 0 0
所以,a1,a2,a3 是一个极大无关组
a4=2a1+a2-a3.