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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

r=3, last term (an) is 486

Sum of terms =Sn=728, a=?

an=486

arn1=486

a(3)n1=486

a(3)n=486×3

a(3)n=1458 ..........(i)

Now Sn=728

728=a(3n131)

728={a(3)na2}

1456=a(3)n

1456=1458a [From (i)]

a=2


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