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Question

Let Sn denote the sum of the first 'n' terms of an A.P and S2n=3Sn. Then the ratio S3n:Sn is equal to

A
4:1
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B
6:1
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C
8:1
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D
10:1
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Solution

The correct option is B 6:1
Sn=n(n+1)2
Now it is is given that S2n=3Sn
2n(2n+1)2=3n(n+1)22(2n+1)=3(n+1)4n+2=3n+34n3n=32n=1
Then S3nSn=3n(3n+1)/2n(n+1)/2=3×41×2=616:1
Option B is the correct answer.

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