已知x :y :z=3:4:7,且2x-y+z= -18,求x+2y-3z的值.

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已知x :y :z=3:4:7,且2x-y+z= -18,求x+2y-3z的值.
已知x :y :z=3:4:7,且2x-y+z= -18,求x+2y-3z的值.

已知x :y :z=3:4:7,且2x-y+z= -18,求x+2y-3z的值.
设x=3a,y=4a,z=7a(a≠0）,
2x-y+z=6a-4a+7a=9a=-18,解得a=-2
所以x=-6,y=-8,z=-14.
x+2y-3z=-6-16+42=20
答毕.

这个比比较打最后是一个个拼出来
X=-6 Y=-8 Z=-14
X+2Y-3Z=20

x/y=3/4 y=(4/3)x
x/z=3/7 z=(7/3)x
2x-y+z=-18 2x-(4/3)x+(7/3)x=-18
x=-6
y=-8
z=-14
20

先设X，Y，Z分别为3K，4K，7K，代入解得9K=-18，K=-2，X，Y，Z分别等于-6，-8，-14代入X+2Y-3Z=20

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