解: 增广矩阵 =
21 -111
42 -212
21 -1 -11
r2-2r1, r3-r1
21 -111
000 -10
000 -20
r1+r2, r3-2r2, r2*(-1)
1 1/2 -1/2 0 1/2
00010
00000
通解为: (1/2,0,0,0)+c1(-1/2,1,0,0)+c2(1/2,0,1,0)