**Ex 9.3, 15**

Last updated at Dec. 8, 2016 by Teachoo

Last updated at Dec. 8, 2016 by Teachoo

Transcript

Ex9.3, 15 Given a G.P. with a = 729 and 7th term 64, determine S7. First term a = 729 and 7th term = 64 we know that nth term of G.P. = arn-1 a7 = ar6 putting values 64 = 729 r6 64/729 = r6 2^6/3^6 = r6 (2/3)^6= r6 (2/3)^6= r6 Comparing powers r = 2/3 We need to find sum of first 7 terms We know that Sum of n terms = ๐(1 โ ๐^๐ )/(1 โ๐) Sn = ๐(1 โ ๐^๐ )/(1 โ๐) S7 = ๐(1 โ๐7)/(1 โ๐) Putting values S7 = 729(1 โ (2/3)^7 )/(1 โ 2/3) S7 = 729(1 โ (2/3)^7 )/(1 โ 2/3) S7 = 729(1 โ 128/2187)/((3 โ 2)/3) S7 = 729(1 โ 128/2187)/(1/3) S7 = 729(2059/2187) ร 3 S7 = 2187(2059/2187) S7 = 2059 Hence S7 = 2059 Hence, sum of first 7 terms is 2059

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Davneet Singh

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He provides courses for Mathematics from Class 9 to 12. You can ask questions here.