[tex]\displaystyle 1+3+5+...+21 \\ 21=1+(n-1) \cdot 2 \\ 21=1+2n-2 \\ 2n=21-1+2 \\ 2n=22 \\ n=22:2 \\ n=11 \\ S_{11}= \frac{2+10 \cdot 2}{2} \cdot 11 \\ \\ S_{11}= \frac{2+20}{2} \cdot 11 \\ \\ S_{11}= \frac{22}{2} \cdot 11 \\ \\ S_{11}=11 \cdot 11 \\ S_{11}=121[/tex]
