Question

Determine a soma dos termos da P.A. (-8, -5, -2, ..22):

Answer 1

a n = 22
r = a 2 - a1
r = -5 - (-8)
r = -5 + 8
r = 3
a n = a 1 + (n - 1).r
22 = -8 + (n - 1).3
22 = -8 + 3 n - 3
22 = -11 + 3 n
22 + 11 = 3 n
33 = 3 n
33/3 = n
n = 11
Sn = n .(a 1 + a n / 2
S 11 = 11 . (-8 + 22) / 2
S 11 = 11.(14) / 2
S 11 = 154 / 2
s 11 = 77