Question

If S_n denote the sum of the first n terms of an A.P. If S_2n = 3S_n, then S_3n : S_n is equal to

 A. 4 

B. 6 

C. 8 

D. 10

Answer 1

We know that Sn =\(\frac{n(n+1)}{2}\)

Now it is given that

S_2n = 3S_n

2(2n + 1) = 3(n + 1) 

4n + 2 = 3n + 3 

n = 1

Now,

= \(\frac {3 \times 4}{2}\)

= 6