x1+x2+x3+x4+x5=0,①x1+2x2+x3+x4-x5=0,②x1+3x2+x3+x4-3x5=0,③3x1+4x2+3x3+3x4+x5=0。④①+②,2x1+3x2+2x3+2x4=0,⑤①*3+③,4x1+6x2+4x3+4x4=0,与⑤同解。④-①,2x1+3x2+2x3+2x4=0,与⑤同解。x5可为任意数,⑤-①*2,x3-2x5=0,x3=2x5,所以x1,x2,x5为任意数,x3=2x5,x4=-x1-x2-3x5.为所求。