Question

Determine a soma dos 21 primeiros termos da pa (12,9,6...)

Answer 1

P.A(12,9,6.....)
r = a 2 - a 1
r = 9 - 12
r = -3
a 21 = a 1 + (n - 1 ).r
a 21 = 12 + (21 - 1 ) - 3
a 21 = 12 + 20 . - 3
a 21 = 12 - 60
a 21 =  - 48
S n = n . (a 1 + a 21 ) / 2
S 21  = 21 (12 + (-48)  / 2
S 21 = 21 ( 12 - 48 ) / 2
S 21 = 21 . -36   / 2
S 21 = -756/2
S 21 = -378