Răspuns :
5+10+15+...+40 /:5 => dam factor comun pe 5 si rezulta
1+2+3+.........+8
Acum folosesti formula n(n+1):2 .
S=1+2+3+..........+8
S=8(8+1):2
S=36
5+10+15+........+40=5*36=180
1+2+3+.........+8
Acum folosesti formula n(n+1):2 .
S=1+2+3+..........+8
S=8(8+1):2
S=36
5+10+15+........+40=5*36=180