Question

Calculati: S=3+6+9+12+.....+192.

Answer 1

S=3×1+3×2+3×4+...+3×64
S=3×(1+2+3+4+...+64)
S=3×(64×65:2)
S=3×(4160:2)
S=3×2080
S=6240

Answer 2

$\displaystyle 3+6+9+12+...+192=3(1+2+3+...+64)=3 \times \frac{64(64+1)}{2} = \\ \\ =3 \times \frac{64 \times 65}{2} =3 \times \frac{4160}{2} =3 \times 2080=6240$