Question

Determine a soma dos 30 primeiros termos de P.A (-15,-11,-7...)

Answer 1

a n = a 1 + (n - 1 ) .r
r = a 2 - a 1
r = -11 - (-15)
r = -11 + 15 = 4
a 30 = - 15 + (30 - 1 ) .4
a 30 = -15 + 29 . 4
a 30 = - 15  + 116
a 30 = 101
S n = n .(a 1 + a n ) /2
S 30 = 30 ( - 15 + 101 )/2
S 30= 30 . 86 / 2
S 30 = 2580/ 2
S 30 = 1290