4x+5y=7 .①
3x-7y=9 .②
①×3,得:
12x+15y=21.③
②×4,得:
12x-28y=36.④
③-④,得:
43y=-15
y=-15/43
同理,①×7+②×5,得:
43x=94
x=94/43
4x+5y+9z=7.①
3x+5y+7z=4.②
2x+7y+9z=6 .③
①-②,得:
x+2z=3
z=(3-x)/2.④
①-③,得:
2x-2y=1
y=x-1/2.⑤
把④,⑤代入①,得:
4x+5(x-1/2)+9(3-x)/2=7
4x+5x-2.5+13.5-4.5x=7
4.5x=-4
x=-8/9
y=-8/9-1/2=-25/18
z=(3+8/9)/2=35/18