\[\boxed{\text{608}\text{\ (608)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ x_{1} = 2,\ x_{2} = 4,\ldots,\ a_{n} = 2n,\ \]
\[S_{n} = \frac{\left( x_{1} + x_{n} \right)}{2} \cdot n = \frac{2 + 2n}{2} \cdot n = n(n + 1) = n^{2} + n.\]
\[\textbf{б)}\ x_{1} = 1,\ x_{2} = 3,\ \ldots,\ x_{n} = 2n - 1,\]
\[S_{n} = \frac{\left( x_{1} + x_{n} \right)}{2} \cdot n = \frac{1 + 2n - 1}{2} \cdot n = n^{2}.\]
\[\ \]