3x+7y+4z=35,x+5y+2z=40,x+y+z=( )

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3x+7y+4z=35,x+5y+2z=40,x+y+z=( )
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3x+7y+4z=35,x+5y+2z=40,x+y+z=( )
3x+7y+4z=35,x+5y+2z=40,x+y+z=( )

3x+7y+4z=35,x+5y+2z=40,x+y+z=( )
3x+7y+4z=35,
x+5y+2z=40
-得
2(x+y+z)=-5
x+y+z=-5/2
完全正确!

3x+7y+4z=35, <1>
x+5y+2z=40 <2>
<1>-<2>得
2(x+y+z)=-5
x+y+z=-5/2

-2.5