2x+y-z=2 x+2y-z=5 x-2y+2z=-7
2x+y-z=2 ①
x+2y-z=5 ②
x-2y+2z=-7 ③
①-2*②得 -3y+z=-8 ④
①-2*③得 5y-5z=16 ⑤
-5*④-3*⑤得 10z=-8 z=-4/5
代入z到④,得y=12/5
代入y和z到①,得x=-3/5
因此 x=-3/5 y=12/5 z=-4/5
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2x+y-z=2 x+2y-z=5 x-2y+2z=-7
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