Question

calculati suma nr :3+6+9+12+15+.....+225

Answer 1

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