Question

3+6+9+......+2016=???

Answer 1

$\displaystyle \mathtt{3+6+9+...+2016=3(1+2+3+...+672)=3 \cdot \frac{672(672+1)}{2}= } \\ \\ \mathtt{=3 \cdot \frac{672 \cdot 673}{2} =3 \cdot 336 \cdot 673=678384}$