[tex]\displaystyle 1+4+7+...+2011+2014 \\ 2014=1+(n-1) \times 3 \\ 2014=1+3n-3 \\ 3n=2014-1+3 \\ 3n=2016 \\ n=2016:3 \\ n=672 \\ S_{672}= \frac{2+671 \times 3}{2} \times 672 \\ \\ S_{672}= \frac{2+2013}{2} \times 672 \\ \\ S_{672}= \frac{2015}{2} \times 672 \\ \\ S_{672}=2015 \times 336 \\ S_{672}=677040[/tex]
