Question

Dado a PA (2,4,..) Calcule : a) a soma dos 7 primeiros termos .b) a soma dos 9 termos.

Answer 1

a₁ = 2

r = 4 - 2 ----- r = 2


A)

a₇ = 2 + (7 - 1) . 2 ----- a₇ = 2 + 6 . 2 ----- a₇ = 2 + 12 ----- a₇ = 14

S₇ = [(2 + 14) . 7]/2 ------ S₇ = [(16) . 7]/2  ------ S₇ = 112/2 ---- S₇ = 56


B)   a₉ = 2 + (9 - 1) . 2 ----- a₉ = 2 + 8 . 2 ---- a₉ = 2 + 16 ---- a₉ = 18

S₉ = [(2 + 18) . 9]/2  ----- S₉ = [(20) . 9]/2 ---- S₉ = [180]/2   ----- S₉ = 90