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If  a1,a2,a3,.......anis an AP with common difference d, then tantan1d1+a1a2+tan1d1+a2a3+....+tan1d1+an1an is equal to 

a
(n−1)da1+an
b
(n−1)d1+a1an
c
nd1+a1an
d
an−a1an+a1

detailed solution

Correct option is B

We have tan−1a2−a11+a1a2+tan−1a3−a21+a2a3+......+tan−1an−an−11+an−1an  ∵d=an-an-1 ∀n                           =(tan−1a2−tan−1a1)+(tan−1a3−tan−1a2)+......+(tan−1an−tan−1an−1)                            =tan−1an−tan−1a1=tan−1an−a11+ana1                             =tan−1(n−1)d1+a1an∴tantan−1d1+a1a2+tan−1d1+a2a3+....+tan−1d1+an−1an=(n−1)d1+a1an

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