[tex]\displaystyle 15+17+19+...+31 \\ 31=15+(n-1) \times 2 \\ 31=15+2n-2 \\ 2n=31-15+2 \\ 2n=18 \\ n=18:2 \\ n=9 \\ S_9= \frac{30+8 \times 2}{2} \times 9 \\ \\ S_9= \frac{30+16}{2} \times 9 \\ \\ S_9= \frac{46}{2} \times 9 \\ \\ S_9=23 \times 9 \\ S_9=207[/tex]
