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Answer:
1026
Step-by-step explanation:
Common difference = d = 2nd term - first term
d = -3 - (-11) = -3 +11 = 8
First term = a = -11
First we need to find the number of terms in this series
[tex]a_{n} = 125\\\\a + (n- 1)d = a_{n}\\\\[/tex]
-11 + (n- 1) * 8 = 125
(n-1)*8 = 125 + 11
(n-1) * 8 = 136
n -1 = 136/8
n -1 = 17
n = 17+1
n = 18
[tex]S_{n} =\frac{n}{2}(a+a_{n})\\\\S_{18} =\frac{18}{2}(-11+125)\\\\[/tex]
= 9 * 114
= 1026