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Question

Let a1,a2,a3...... be in harmonic progression with a1=5 and a20=25. The least positive integer n for which an<0 is


Your Answer
A
22
Your Answer
B
23
Your Answer
C
24
Correct Answer
D
25

Solution

The correct option is A 25
a1,a2,a3,.... are in H.P.
1a1,1a2,1a3....... are in A.P.
1a1=15 and 1a20=125
1a1+19d=1a2015+19d=125d=4475
Now 1an=15+(n1)(4475)
Clearly an<0 if 1an<0154n475+4475<0
4n<99 or n>994=2434n25
Hence least value of n is 25.

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