Question 1180187
the sum of the arithmetic serie whose first two terms are -12 and -8, respectively, and whose last term is 124.

-12, -8, ...........124

a= -12, d=4, tn =124

tn = nth term = a+(n-1)d

124 = -12+(n-1)4

136 = 4n-4
140 = 4n
n= 35 There area 35 terms

Sum of terms Sn = (n/2) * (a+l) where a is the first term and l is the last term

Sn = (35/2) (-12+124)

= 35*112/2

35* 56 =1960

sum of the arithmetic series = 1960