Question

Let Sn denote the sum of first n terms of an A.P. If S2n = 3Sn, then S3n : Sn is equal to​

Answer 1

We know that Sn = n(n+1)/2

now it is given that

S2n = 3Sn

=> 2n(2n+1)/2 = 3n(n+1)/2

=> 2(2n+1) = 3(n+1)

=> 4n+2 = 3n+3

=> n = 1

then S3n/Sn = [3n(3n+1)/2]/[n(n+1)/2] = 3*4/1*2 = 6

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