Question

Aflati suma
:S= 5+10+15+20+...+125

Answer 1

$\displaystyle 5+10+15+20+...+125=5(1+2+3+...+25)= \\ \\ =5 \times \frac{25(25+1)}{2} =5 \times \frac{25 \times 26}{2} =5 \times \frac{650}{2} =5 \times 325=1625$

Answer 2

$S=5+10+15+20+...+125= \\ =\ \textgreater \ 5*(1+2+3+...+25) \\ =\ \textgreater \ 5* \frac{25*(25+1)}{2} \\ =\ \textgreater \ 5* \frac{25*26}{2} \\ =\ \textgreater \ 5* \frac{650}{2} \\ =\ \textgreater \ 5*325 \\ =\ \textgreater \ \boxed{1625}$