Question

S=3+6+9+12+15+....+225

Answer 1

Dam factor cumun pe 3
3(1+2+3+4+5+...+75)
Aplicam suma lui Gauss
3[75(75+1)/2]=3(75 x 76)/2=3 x 5700/2=8550

Answer 2

$\displaystyle S=3+6+9+12+15+...+225 \\ \\ S=3(1+2+3+4+5+...+75) \\ \\ S=3 \times \frac{75(75+1)}{2} \\ \\ S=3 \times \frac{75 \times 76}{2} \\ \\ S=3 \times \frac{5700}{2} \\ \\ S=3 \times 2850 \\ \\ S=8550$