\(P=\frac{x}{y+z}+\frac{y}{z+x}+\frac{z}{x+y}\)
\(P=\frac{x^2}{xy+xz}+\frac{y^2}{zy+xy}+\frac{z^2}{xz+zy}\)
\(P\ge\frac{\left(x+y+z\right)^2}{2\left(xy+yz+zx\right)}\)(cauchy-schwarz)\(\ge\frac{3\left(xy+yz+zx\right)}{2\left(xy+yz+zx\right)}=\frac{3}{2}\)
"="<=>x=y=z