Question

Calcule a soma dos doze primeiros termos da pa -15,-11,-7, ...

Answer 1

P.A.(-15,-11,-7,...)
a 1  = -15
a 2 = -11
r = a 2 - a
r =  -11 - (-15)
r = -11 + 15
r = 4
a 12 = a 1 + (n - 1 ).r
a 12 = -15 + (12 - 1 ).4
a 12 = -15 + 11.4
a 12 = -15 + 44
a 12 = 29
S 12 = 12.(A 1 + A 12 ) / 2
S 12 = 12.(-15 + 29) / 2
S 12 = 12 (14) / 2
S 12 = 168 / 2
S 12 = 84 

Answer 2

an= a1+ (n-1) . r
an= -15 + (12-1) . 4
an= -15 + 11 . 4
an= -15 + 44
an= 29