Question

Calcule a soma dos 25 termos iniciais da P.A. (1,7,13,...)

Answer 1

a1 = 1   n = 25   r = 6

a25 = 1 + ( 25 - 1 ) . 6
a25 = 1 +  24 . 6
a25 = 1 + 144
a25 = 145

Answer 2

$a_{n}$ = $a_{1}$ + (n - 1)r
$a_{n}$ = 1 + (25 - 1)6
$a_{n}$ = 1 + 24 * 6
$a_{n}$ = 1 + 144
$a_{n}$= 145

S = {145}