x+2y+z=2-----(1) x-4y+z=5-----(2) 2x-2y-5z=0-----(3)
(1)*2+(2)得：3x+3z=9-----x+z=3-----(4) 并代入(1)或(2) 得：y=-1/2
代入(3)得：2x-5z=-1-----(5) (4)*5+(5)得：7x=14 ==> x=2
代入(4)得：z=1 ∴x=2,y=-1/2,z=1.