Question

caculati sumele..
2+7+12+................+177

Answer 1

$\displaystyle 2+7+12+...+177 \\ \\ 177=2+(n-1) \times 5 \\ \\ 177=2+5n-5 \\ \\ 5n=177-2+5 \\ \\ 5n=180 \\ \\ n= \frac{180}{5} \\ \\ n=36$

$\displaystyle S_{36}= \frac{4+35 \times 5}{2} \times 36 \\ \\ S_{36}= \frac{4+175}{2} \times 36 \\ \\ S_{36}= \frac{179}{2} \times 36 \\ \\ S_{36}=179 \times 18 \\ \\ S_{36}=3222$