If in a G.P. of 64 terms, the sum of all the terms is 7 times the sum of the odd terms of the G.P, then the common ratio of the G.P. is equal to
- 7
- 6
- 5
- 4
Answer
(2) 6
Solution
Let the G.P. be a, ar, ar2, , ar63. The odd-placed terms a, ar2, , ar62 are 32 terms of a G.P. with common ratio r2.
Using the G.P. sum formula for both sums (r 1, since those values cannot satisfy the condition):
a(r64 - 1)r - 1 = 7 a(r64 - 1)r2 - 1
Since a(r64 - 1) 0, cancelling it gives r2 - 1 = 7(r - 1), i.e. r + 1 = 7.
∴ r = 6
Answer: (2)