For $x_i\ge0$, where $i=1,2,...,n$, satisfying
$$\sum_{i=1}^n\,x_i^2+2\,\sum_{1\le k find the maximum and minimum of $\sum_{i=1}^n x_i.$
For $x_i\ge0$, where $i=1,2,...,n$, satisfying
$$\sum_{i=1}^n\,x_i^2+2\,\sum_{1\le k find the maximum and minimum of $\sum_{i=1}^n x_i.$