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Question

If a1,a2,a3,...,an are in A.P of non-zero terms, Prove that 1a1a2+1a2a3+1a3a4+...1an1an=n1a1an

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Solution

Let d be the common difference of the A.P
d=a2a1=a3a2=a4a3=...=anan1

Given 1a1a2+1a2a3+1a3a4+...1an1ann1a1an

=1d(da1a2+da2a3+da3a4+...dan1ann1a1an)

=1d(a2a1a1a2+a3a2a2a3+a4a3a3a4+...anan1an1ann1a1an)

=1d(a2a1a2a1a1a2+a3a2a3a2a2a3+...+anan1anan1an1an)

=1d[1a11a2+1a21a3+...+1an11an]

=1d[1a11an]

=1d[ana1a1an]

=1d[a1+(n1)da1a1an]

=n1a1an

Hence proved.

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