The sum of two terms of gp is 6 and that of first four terms is 15/2.Find the sum of first six terms.

Given:

The sum of two terms of GP is 6 and that of first four terms is

To find:

The sum of first six terms.

Solution:

We have,

Sum of first n terms of a GP is

             ...(i)

Putting n=2, we get

                   ...(ii)

Putting n=4, we get

           (Using (ii))

Divide both sides by 6.

Taking square root on both sides, we get

Case 1: If r is positive, then using (ii) we get

The sum of first 6 terms is

Case 2: If r is negative, then using (ii) we get

The sum of first 6 terms is

Therefore, the sum of the first six terms is 7.875.

answer: c. is your answer

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