If x = uv and \({\rm{y}} = \frac{{{\rm{u}} + {\rm{v}}}}{{{\rm{u}} - {\rm{v}}}}\), then \(\frac{{{\rm{\delta }}\left( {{\rm{u}},{\rm{\;v}}} \right)}}{{{\rm{\delta }}\left( {{\rm{x}},{\rm{\;y}}} \right)}}\) is a function of f(u,v). Find the value of f(2,1).