If a1,a2,a3,....,an are in AP , then 1a1a2+1a2a3+1a3a4+.....+1an−1an equals.
A
a1a2n−1
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B
n−1a1+an
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C
n−1a1−an
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D
n−1a1an
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Solution
The correct option is Dn−1a1an If a1,a2,a3,...........................an−1,an are in A.P then common difference d=an−an−1=........=a3−a2=a2−a1....(1) Thus 1a1a2+1a2a3+1a3a4+.....+1an−1an =1d(a2−a1a1a2+a3−a2a2a3+a4−a3a3a4+.....+an−an−1an−1an) using (1) =1d(1a1−1an)=1dan−a1a1an=n−1a1an Hence, option 'D' is correct.