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Find the sum of the geometric sequence –3, 15, –75, 375, … when there are 8 terms and select the correct answer below.

a.) –195,312
b.) 195,312
c.) 292,968
d.) –292,968

I need help i had solved this question and i got the answer b but i want to make sure i am correct.

User Rabbits
by
8.1k points

1 Answer

2 votes

Answer: Option 'B' is correct.

Explanation:

Since we have given that

–3, 15, –75, 375, …

here, a = first term = -3

r = common ratio is given by


(a_2)/(a_1)=(15)/(-3)=-5

Number of terms = n =8

As we know the "Sum of geometric sequence":


a_n=(a(r^n-1))/(r-1)\\\\a_8=(-3(-5^8-1))/(-5-1)\\\\a_8=-3(390625-1)}{-6}\\\\a_8=195,312

Hence, Option 'B' is correct.

User Zoltan Ersek
by
8.0k points
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