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

Here, we are given an A.P. whose sum of n terms is S_n and.

We need to find.

Here we use the following formula for the sum of n terms of an A.P.,

Where; a = first term for the given A.P.

d = common difference of the given A.P.

n = number of terms

So, first we find S_3n,

Similarly,

Also,

Now,

So, using (2) and (3), we get,

On further solving, we get,

So,

Taking common, we get,

Therefore,

Hence, the correct option is (b).