CAT 2003 Slot 1 — QA Question 40
Arithmetic ProgressionEasy
If log3 2, log3 (2x − 5), log3 (2x − 7/2) are in arithmetic progression, then the value of x is equal to
Answer & solution
Correct answer: 3
- A
5
- B
4
- C
2
3
Solution
log3 2, log3 (2x − 5), log3 (2x − 7/2) are in A.P.
∴ 2 × log3 (2x − 5) = log3 2 + log3 (2x − 7/2)
∴ log3 (2x − 5)2 = log3 [2 × (2x − 7/2)]
Let 2x = a, then we have,
(a − 5)2 = 2 × (a − 7/2)
∴ a2 − 10a + 25 = 2a − 7
∴ a2 − 12a + 32 = 0
∴ a2 − 8a − 4a + 32 = 0
(a − 8)(a − 4) = 0
a = 8 or 4
2x = 8 or 2x = 4
x = 3 and x = 2
x = 2 cannot be the answer as (2x − 5) would become negative and logarithms of negative numbers are not defined.
∴ x = 3
Hence, option (d).