= (x+y-z)/(x+y+z)

p^2 =(y+z-x)/(x+y+z)*(z+x-y)/(y+z-x)

= (z+x-y)/(z+x+y)

p = (y+z-x)/(x+y+Z)

p^3+p^2+p=(x+y-z+z+x-y+z-x+y)/(z+x+y)

= 1