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Question

The common ratio of a G.P. is 3 and the last term is 486. If the sum of these terms be 728, find the first term.

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Solution

Here, common ratio, r = 3
nth term, an = 486
Sn = 728

an=486 arn-1=486a3n-1 = 486 a3n = 486 × 3 a3n =1458 ... iNow, Sn = 728728 = a 3n-13-1 728 = a3n - a21456 = a3n-1 - a 1456= 1458- a From ia = 1458 - 1456 a= 2

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