Question

If Sn denote the sum of the first n terms of an A.P. If S₂n = 3Sn, then S₃n : Sn is equal to
A. 4
B. 6
C. 8
D. 10

Answer 1

B. 6

Step-by-step explanation:

Given: S₂n = 3Sn

Find S₃n : Sn

Solution:

 We know that Sn = n(n+1)/2

 So when S2n = 3Sn

 2n(2n+1)/2 = 3n(n+1)/2

 2(2n+1) = 3(n+1)

 4n+2 = 3n+3

 n = 1

Now S3n/Sn = [3n(3n+1)/2]/[n(n+1)/2]  

  = 3(3n+1)/ (n+1)  

  = 3*4 / 2  

  = 6

Option B is the answer.