[tex] S_{10}= \frac{( a_{1} + a_{n})*n}{2} = \frac{(2* a_{1} + (n-1)*r)*n}{2}= a_{1} * n + r * \frac{n(n-1)}{2} \\ 245= a_{1} * 10 + 5* \frac{10(10-1)}{2} =\ \textgreater \ \\ =\ \textgreater \ 245 = 10a_{1} + 5 *5*9 =\ \textgreater \ 245= 10a_{1} + 225 =\ \textgreater \ 245-225=10a_{1} \\ 20= 10a_{1} | :10 =\ \textgreater \ a_{1} = 2[/tex]
