Q5 of 110 Page 58

How many consecutive terms starting from the first term of the series

(i) 3 + 9 + 27 + … would sum to 1092 ? (ii) 2 + 6 + 18 + … would sum to 728 ?


(i) First term = 3

Common ratio = = 3


Sum of n terms =


⇒ Sum of n terms =


⇒ 1092 =


⇒ 728 =


⇒ 729 =


⇒ n = 6


(ii) First term = 2


Common ratio = = 3


Sum of n terms =


⇒ 728 =


⇒ 728 =


⇒ 729 =


⇒ n = 6


More from this chapter

All 110 →
3

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

 4

Find the sum of the following finite series

(i) 1 + 0.1 + 0.01 + 0.001 + … + ( 0.1)9 (ii) 1 + 11 + 111 + … to 20 terms.

 6

The second term of a geometric series is 3 and the common ratio is . Find the sum of

first 23 consecutive terms in the given geometric series.

 7

A geometric series consists of four terms and has a positive common ratio. The sum of the first two terms is 9 and sum of the last two terms is 36. Find the series.