Question

Calcular a soma dos 80 primeiros termos da PA ( 6,9,12,15,16.....)

Answer 1

r = a₂ - a₁
r = 9 -6
r = 3

$A_{n} = a_{1} + (n -1) . r$
a80 = 6 + 79 . 3
a80 = 243

$S_{n} = \frac{ ( a_{1} + a_{n} ) . n}{2}$
s80 = (6 + 243) .80/2
s80 = 249 . 40
s80 = 9960