Question

3+6+9+...+90 repedeee

Answer 1

=3×1+3×2+3×3+...+3×30=

=3(1+2+3+...+30)=

$=3*\frac{30*31}{2}=$

=3×15×31=

=45×31=

$\boxed{=1.395}$

Answer 2

3+6+9+...+90=
3(1+2+3+...+30)=
3·30·31 : 2 = 2790 : 2
                  = 1395