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Q45 of 74 Page 9

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

Given : 3 + 3² + 3³+ ….. = 120

Now a = 3, r = 3. Let number of terms = n

Also = 120
or = 120

or 3ⁿ – 1 =

or 3ⁿ – 1 = 80

or 3ⁿ = 81 = 3⁴

Hence n = 4

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