Question

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

Answer 1

PA (-8, -5, -2... 22)
a1 = -8
r = -5 - (-8) = -5 + 8 = 3

an = a1 + (n - 1) . r
22 = -8 + (n - 1) . 3
22 = -8 + 3n - 3
22 + 8 + 3 = 3n
33 = 3n
33/3 = n
11 = n

Sn = (a1 + an) . n/2
S11 = (-8 + 22) . 11/2
S11 = 14 . 11/2
S11 = 154/2 = 77