\[\boxed{\text{603}\text{\ (603)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[S_{n} = \frac{\left( a_{1} + a_{n} \right)}{2} \cdot n;\]
\[\textbf{а)}\ a_{1} = 3;\ \ a_{60} = 57:\]
\[S_{60} = \frac{(3 + 57)}{2} \cdot 60 = 60 \cdot 30 = 1800.\]
\[\textbf{б)}\ a_{1} = - 10,5;\ \ a_{60} = 51,5:\]
\[S_{60} = \frac{( - 10,5 + 51,5) \cdot 60}{2} = 41 \cdot 30 = 1230.\]