Question

Repede va rog 5+10+15... + 300 =

Answer 1

$\displaystyle 5+10+15+...+300=5(1+2+3+...+60)= 5 \cdot \frac{60(60+1)}{2} = \\ \\ =5 \cdot \frac{60 \cdot 61}{2} =5 \cdot \frac{3660}{2} =5 \cdot 1830=9150$