Question

Efectuați?
5+10+15+•••+95

Answer 1

$\displaystyle 5+10+15+...+95=5(1+2+3+...+19)=5 \times \frac{19(19+1)}{2} = \\ \\ =5 \times \frac{19 \times 20}{2} =5 \times \frac{380}{2} =5 \times 190=950$