If b1,b2,b3,...bn are in H.P then b1b2+b2b3+b3b4+...+bn−1bn=
A
(n+1)b1bn
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B
nb1bn
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C
(n+1)bnbn−1
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D
(n−1)b1bn
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Solution
The correct option is A(n−1)b1bn 1b1−1b2=d ⇒b1−b2b1b2=d ⇒b1−b2d=b1b2,...bn−1−bnd=bn−1bn ⇒b1−b2d+b2−b3d+b3−b4d+...+bn−1−bnd=b1b2+b2b3+b3b4+...+bn−1bn...(1) Also, (n−1)=b1−bnb1bn....(2) From eqns(1) and (2) ∴b1b2+b2b3+b3b4+...+bn−1bn=(n−1)b1bn