Question

determine a soma dos 18 termos da pa 1,4,7....16

Answer 1

$an=a1+(n-1)r$
$a18=1+17r$
$a18=1+17.3$
$a18=1+51$
$a18=52$
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$Sn= \frac{(a1+an).n}{2}$
$S18= \frac{(a1+a18).18}{2}$
$S18= \frac{(1+52).18}{2}$
$S18= \frac{53.18}{2}$
$S18= \frac{954}{2}$
$S18=477$