Hard Maths 2

u=(p+q)/(q-r)=-2x/(x+y)= -2+ 2y/(x+y)
∂u/∂x=-2y/(x+y)^2
∂u/∂y=2x/(x+y)^2
(∂/∂x)^2 u= 4y/(x+y)^3
(∂/∂y)^2 u= -4x/(x+y)^3
(∂/∂x)^2 u+(∂/∂y)^2 u =-4(x-y)/(x+y)^3 =0, then x=y(≠0)
so, u=-2x/(x+y)= -1