Răspuns :
Vom folosi formula sumei lui Gauss:
[tex]1+2+3+...+n=\frac{n(n+1)}{2}[/tex]
[tex]S=3+6+9+...+90\\ S=3\cdot1+3\cdot2+3\cdot3+...+3\cdot30\\ S=3(1+2+3+...+30)\\ S=3\cdot\frac{30\cdot31}{2}=\boxed{1395}[/tex]
[tex]1+2+3+...+n=\frac{n(n+1)}{2}[/tex]
[tex]S=3+6+9+...+90\\ S=3\cdot1+3\cdot2+3\cdot3+...+3\cdot30\\ S=3(1+2+3+...+30)\\ S=3\cdot\frac{30\cdot31}{2}=\boxed{1395}[/tex]