X1-X2-X3+X4=0(1)X1-X2+X3-3X4=1(2)X1-X2-2X3+3X4=-1/2(3)。(2)-(1),得2x3-4x4=1,(2)-(3),得3x3-6x4=3/2.上述两个方程同解,所以x3=(4x4+1)/2,x4可以是任意数;代入(1),得x1=x2+(2x4+1)/2,x2可以是任意数。