Question

Efectuati calculele:
a) 3+6+9+...+99-(1+4+7+...+97)=?

Cat se poate de rapid ! Va rog !​

Answer 1

=3(1+2+3+...+11)-(1+4+7+...+97)

=3× (11×12)/2 - [97×(3×97-1)]/2

=3×132/2 - (97×290)/2

=3×66-14065

=13068-14065

=-997

*1+2+3+...+n=[n(n+1)]/2

1+4+7+...+3n-2=[n(3n-1)]/2