X1+X2+2X2-X4=0打错，应该是X1+X2+2X3-X4=0┏1 1 2 -1┓┃-1 0 -3 2┃┗2 1 5 -3┛→﹙行初等变换﹚→┏1 0 3 -2┓┃0 1 -1 1┃┗0 0 0 0┛通解﹛x1,x2,x3,x4﹜＝a﹛-3,1,1,0﹜+b﹛2,-1,0,1﹜ [a.b是任意数]
