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Q. If a1,a2,a3,..............an are in harmonic progression, then a1a2+a2a3+....+an-1an is equal to. 
(a) (n-1)a1an                  (b) na1an
(c) n(a1+an)                   (d) None of these.

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Ans = ( a ) (n -1) a1an .

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