If \(u\left( {x,\;y} \right) = {x^4}{\tan ^{ - 1}}\left( {\frac{y}{x}} \right) + \left( {\frac{{{x^5}\; + \;{y^5}}}{{x\; + \;y}}} \right),\) then \(x\frac{{\partial \upsilon }}{{\partial x}} + y\frac{{\partial \upsilon }}{{\partial y}}\) is.