Question

calcule a soma dos 8 primeiros termos da pa 1,2, 3,4

Answer 1

$an=a1+(n-1)r$
$a8=a1+7r$
$a8=1+7.1$
$a8=8$

$Sn= \frac{(a1+an).n}{2}$
$S8= \frac{(a1+a8).8}{2}$
$S8= \frac{(1+8).8}{2}$
$S8= \frac{9.8}{2}$
$S8= \frac{72}{2}$
$S8=36$