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15 votes
15 votes
The sum of the first 15 terms of a geometric sequence with 7 as the first term and a common ratio of -3 is __________.

User Ronnette
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1 Answer

10 votes
10 votes

Explanation:

the formula for the sum of the first n terms of a geometric sequence is

Sn = s1(1 - r^n)/(1 - r)

with r being the common ratio and s1 is the first term.

so,

S15 = 7×(1 - (-3)¹⁵)/(1 - -3) = 7×(1 - -14,348,907)/4 =

= 7×14,348,908/4 = 7×3,587,227 = 25,110,589

User Slosd
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