1 Answer

Narandhran Thangavel · Answer 1 · 2019-12-21T21:38:12+0000

Given geometric sequence = -3,15,-75,375,....

Common ratio = r =
(15)/(-3) = -5

$S_(n)= a\left ( (1-r^(n))/(1 - r) \right )$

To find the sum of seven terms,

n = number of terms = 7

a = 1st term = -3

r = common ratio = -5

$S_(7)= (-3)\left ( (1-(-5)^(7))/(1 - (-5)) \right )$

$S_(7)= (-3)\left ( (1+78125)/(1 + 5) \right )$

$S_(7)= (-3)\left ( (78126)/(6) \right )$

S_(7)= (-3)* 13021

S_(7) = - 39063

Sum of 7 terms of the geometric sequence = - 39063

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