CAT 2017 Slot 1 — QA Question 31
Arithmetic ProgressionEasy
If the square of the 7th term of an arithmetic progression with positive common difference equals the product of the 3rd and 17th terms, then the ratio of the first term to the common difference is:
Answer & solution
2 : 3
- B
3 : 2
- C
3 : 4
- D
4 : 3
Solution
Let the first term be a and the common difference be d.
(a + 6d)2 = (a + 2d) (a + 16d)
⇒ a2 + 12ad + 36d2 = a2 + 18ad + 32d2
⇒ 4d2 = 6ad
⇒ a/d = 2/3
Hence, option (a).