构造矩阵(a1,a2,a3,a4)
用初等行变换化成(字数受限,
1 0 3/2 1
0 1 -7/2 2
0 0 0 0
极大无关组为 a1,a2
且 a3=(3/2)a1-(7/2)a2
a4 = a1+2a2