- Ta có :
`(x-y)+(y-z)+(z+x)=-9+(-10)+11`
`<=>x-y+y-z+z+x=-19+11`
`<=>(x+x)+(-y+y)+(-z+z)=-8`
`<=>2x+0+0=-8`
`<=>2x=-8`
`<=>x=-8:2`
`<=>x=-4`
- Thay `x=-4` có :
`x-y=-9`
`<=> -4-y=-9`
`<=> -4+9=y`
`<=> 5=y`
`<=> y=5`
`z+x=11`
`<=> z+(-4)=11`
`<=> z=11+4`
`<=> z=15`
- Vậy $\begin{cases} x=-4 \\ y=5 \\ z=15 \\ \end{cases}$