Q15 of 97 Page 192

Given a G.P. with a = 729 and 7th term 64, determine S7.

Given: a = 729 and a7 = 64.


nth term of a G.P is given by arn-1


∴ a7 = ar7-1


⇒ 64 = 729 × (r)6


⇒ r6 =


⇒ r6 =


∴ r =


The Sum of terms in G.P. is given by: (∵ r < 1)



⇒ 37 – 27 = 2187 – 128 = 2059


∴ S7 = 2059


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