\[\begin{array}{l} {\text{Докажите тождество:}} \hfill \\ {{\text{1}}^2} + {2^2} + {3^2} + ... + {n^2} = \frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}{6}. \hfill \\ \end{array}\]