[tex]\displaystyle 1+3+5+...+35 \\ 35=1+(n-1) \times 2 \\ 35=1+2n-2 \\ 2n=35-1+2 \\ 2n=36 \\ n=36:2 \\ n=18 \\ S_{18}= \frac{2+17 \times 2}{2} \times 18 \\ \\ S_{18}= \frac{2+34}{2} \times 18 \\ \\ S_{18}= \frac{36}{2} \times 18 \\ \\ S_{18}=18 \times 18 \\ S_{18}=324[/tex]
