Câu trong ảnh làm như nào các bạn ơi...

Ta có:
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
\frac{x}{y+z+1} =\frac{y}{x+z+1} =\frac{z}{x+y-2} =\frac{x+y+z}{y+z+1+x+z+1+x+y-2}\\
\Rightarrow \frac{x}{y+z+1} =\frac{y}{x+z+1} =\frac{z}{x+y-2} =\frac{x+y+z}{2( x+y+z)} =\frac{1}{2} =x+y+z\\
\Rightarrow \begin{cases}
2x=y+z+1\\
2y=x+z+1\\
2z=x+y-2\\
x+y+z=\frac{1}{2}
\end{cases}\\
\Rightarrow y+z=\frac{1}{2} -x\\
\Rightarrow 2x=\frac{1}{2} -x+1\\
\Leftrightarrow x=\frac{1}{2}\\
\Rightarrow \begin{cases}
y+z=0 & \\
2y=z+\frac{3}{2} & 
\end{cases}\\
\Rightarrow y=-z\\
\Rightarrow -3z=\frac{3}{2} \Leftrightarrow z=\frac{-1}{2}\\
\Rightarrow y=\frac{1}{2}
\end{array}$