Aplucam suma lui Gauss.
9+10+11+...+30=
1+2+3+...+9+10+11+...+30-1+2+3+...+8
[30×(30+1):2]-[8×(8+1):2]=
(30×31:2)-(8×9:2)=
(930:2)-(72:2)
465-36=429
Sper ca e bine si ca te-am ajutat!!! SUCCES