Question

1+5+9+13+.......+165?

Answer 1

1+5+9+13+...+165= [(165+1) × 42] : 2 = (166×42):2=6972:2=3486

n=(165-1):4 + 1= 164:4 +1 = 41+1=42 (Numarul de numere)

$Suma=\frac{(u+p)*n}{2}\\\\n=\frac{(u-p)}{r}+1\\\\u=ultimul\ termen\ al\ sumei\\\\p=primul\ termen\ al\ sumei\\\\n=numarul\ de\ termeni\\\\r=ratia/ din\ in\ cat\ sunt\ (1,5,9,...\ =>\ r=4)$