Question

61. Determine a soma dos termos de cada PA
a) (3,11,19,...,115)

Answer 1

An = a1 + (n-1).r
115 = 3 + (n -1).8
115 = 3 + 8n - 8
115 + 5 = 8n
120 = 8n
n = 15

Soma: (a1 + an).n/2
Soma = (3 + 115). 15/2
Soma = 118.15/2
Soma = 885