The Symbols $\left[\sum_{i=1}^{n} x_{i}\right]^{2}$ And $\sum_{i=1}^{n} x_{i}^{2}$ Are Always Equal

The symbols $\left[\sum_{i=1}^{n} x_{i}\right]^{2}$ and $\sum_{i=1}^{n} x_{i}^{2}$ are always equal.