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Question

If a1,a2,a3,....,an are in AP , then 1a1a2+1a2a3+1a3a4+.....+1an−1an equals.

A
a1a2n1
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B
n1a1+an
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C
n1a1an
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D
n1a1an
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Solution

The correct option is D n1a1an
If a1,a2,a3,...........................an1,an are in A.P
then common difference d=anan1=........=a3a2=a2a1....(1)
Thus 1a1a2+1a2a3+1a3a4+.....+1an1an
=1d(a2a1a1a2+a3a2a2a3+a4a3a3a4+.....+anan1an1an) using (1)
=1d(1a11an)=1dana1a1an=n1a1an
Hence, option 'D' is correct.

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