2x+3y-z=11 ...(1)
2x+y-5z=8 ...(2)
-2x+7y+z=19 ...(3)
(1)+(3) 得y=3;
(1)-(2) 得2y+4z=3;
将y=3代入上式得 z=-3/4;
把y和z代入（1）得x=5/8l