x-y-z=0 .1x+y-3x=4 .22x-5y+7z=21 .31+2=(x+y-3z)+(x-y-z)=2x-4z=4 所以 x-2z=2.41-2=(x+y-3z)-(x-y-z)=2y-2z=4 所以Y-z=2.5将4和5【 x-2z=2和Y-z=2】代入3【 2x-5y+7z=21】,解得z=27,y=29.x=56