Q13 of 97 Page 192

How many terms of G.P. 3, 32, 33, … are needed to give the sum 120?

Given: G.P. 3, 32, 33, … and Sum = 120


Here a = 3 and r = = 3


The Sum of terms in G.P. is given by:


∴ = 120


⇒ = 120


⇒ = 120(2)


⇒ = 240


⇒ = = 80


⇒ 3n = 80 +1 = 81


⇒ 3n = 81


Applying ln on both sides, we get



⇒


⇒


⇒ n = 4


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