Sum of first 10 terms of a GP is equal to the sum of the first 12 terms in the…
2020
Sum of first 10 terms of a GP is equal to the sum of the first 12 terms in the same GP. Sum of the first 15 terms is 31, what is the third term in the GP?
Answer: B. 31 — S10 = S12 means the 11th and 12th terms together are zero. So ar¹⁰ + ar¹¹ = ar¹⁰(1 + r) = 0, giving r = -1. With r = -1, S15 = a because the sum of 15…
- A.
-21
- B.
31
- C.
0
- D.
-31
Attempted by 1 students.
Show answer & explanation
Correct answer: B
S10 = S12 means the 11th and 12th terms together are zero.
So ar¹⁰ + ar¹¹ = ar¹⁰(1 + r) = 0, giving r = -1.
With r = -1, S15 = a because the sum of 15 alternating terms leaves one a.
Given S15 = 31, a = 31. The third term is ar² = 31.
Explore the full course: Iocl Engineers Officer Grade A Paper 1
Loading lesson…