Question

Calcular o trigésimo termo da p.a (3,7,11,...)

Answer 1

$S_{n} = \frac{n}{2} [2 a_{1}+(n-1)*d ]$
$S_{30} = \frac{30}{2} [2*3+(30-1)*4 ]$
$S_{30} = 1830$

Answer 2

a1 = 3
a2 = 7
a3 = 11
r = 7 - 3 = 4 ****

a30  = a1 + 29r =  3 +  29 ( 4 )  = 3 + 116 = 119 ****