Question

Determine a soma dos 30 primeiros termos da p.a ( -5 , -2 , 1.. )

Answer 1

P.A.(-5,-2,1,....)
r = a 2 - a 1
r = -2 - (-5)
r = -2 + 5
r = 3

a 30 = a 1 + (n - 1 ).r
a 30 = -5 + (30 - 1 ).3
a 30 = -5 + 29 . 3
a 30 = -5 + 87
a 30 = 82

S n = n.(a 1 + a n) / 2
S 30 = 30 . (-5 + 82 ) / 2
S 30 = 30.77 / 2
S 30 = 2310 / 2
S 30 = 1155