Q3 of 110 Page 58

Find Snfor each of the geometric series described below.

(i) a = 3, t8 = 384, n = 8. (ii) a = 5, r = 3, n = 12.


Ans. 765


(i) t8 = 384

⇒ ar8–1 = 384


⇒ (3)r7 = 384


⇒ r7 = 128


⇒ r = 2


Sum of n terms =


⇒ Sum of 8 terms =


⇒ Sum of 8 terms = 3(255) = 765


(ii) Sum of n terms =


⇒ Sum of 12 terms =


⇒ Sum of 12 terms =


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