Question

suma lui Gauss la 7+14+21+...+210

Answer 1

$7+14+21+...+210= 7(1+2+3+....+30) \\\\\\ S= 7* \frac{\not 30*(1+30)}{\not 2} \\\\ S= 7*15*31 \\\\ \boxed{S=3255}$

Answer 2

Il dam pe 7 factor comun:

7 x( 1+2+3+....+30)

formula: $\frac{n(n+1)}{2}$
n=30
7x$\frac{30x(30+1)}{2}$= 7x$\frac{30x31}{2}$= 7x 456=3255                         
Nu sunt sigura, dar cred ca asa se procedeaza! Sper sa fie bine!