$$u_{c}^{2}(y)=\sum_{i=1}^{n}\left (\frac{\partial f}{\partial x_{i}} \right )^2u^{2}(x_{i})+2\sum_{i=1}^{n-1}\sum_{j=i+1}^{n}\frac{\partial f}{\partial x_{i}}\frac{\partial f}{\partial x_{j}}u\left ( x_{i},x_{j} \right )$$