Question

Calculați : 3 + 6 +9 + ....... +99 =

Answer 1

...=3*(1+2+3+...+33)=3 * (33*34:2)=99*17=1683

Answer 2

$\displaystyle 3+6+9+...+99=3(1+2+3+...+33)=3 \times \frac{33(33+1)}{2} = \\ \\ =3 \times \frac{33 \times 34}{2} =3 \times \frac{1122}{2} =3 \times 561=1683$