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If a, b and c in G.P., Prove that : log an, log bn and log cn are in A.P. -

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Question

Q 5

Sum

Solution

Let the first of the G.P. be a and its common ratio be r.
Then,
(p+q)th term = m ⇒ arp+q-1 = m

(p-q)th term = n ⇒ arp-q-1 = n

Now,
m x n = arp+q-1 x arp-q-1 

= a2 x rp+q-1+p-q-1
= a2 x r2p-2
= a2 x r2(p-1)
= (a x rp-1)2

⇒ arp-1 = `sqrt"mn"`

⇒ pth term = tp = `sqrt"mn"`

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Simple Applications - Geometric Progression
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