a=(1+2+3+4+...+2015)-(11+12+13+...+2015) si cu demonstratie va rog !
repede ca nu am timp !


Răspuns :

1+2+3+4+5+6+7+8+9+10+11+12+13....+2015 - 11-12-13-...-2015 =
1+2+3+....+10=10(10+1)/2=10*11/2=55  (am utilizat n(n+1)/2)