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Question

If a1,a2,a3,. an are in AP with common difference 5 and if aiaj-1fori,j=1,2,.N, then tan-15[1+a1a2]+tan-15[1+a2a3]+.+tan-15[1+an-1an]=


A

tan-15[1+an-1an]

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B

tan-15a1[1+ana1]

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C

tan-15n-5[1+ana1]

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D

tan-15n-5[1+an+1a1]

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E

tan-15n[1+ana1]

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Solution

The correct option is C

tan-15n-5[1+ana1]


Explanation for the correct option:

Finding the value of tan-15[1+a1a2]+tan-15[1+a2a3]+.+tan-15[1+an-1an]:

Given a1,a2,a3,.are in AP with common difference d=5

a2-a1=a3-a2=a4-a3=.....=an-an-1=5tan-15[1+a1a2]+tan-15[1+a2a3]+.+tan-15[1+an-1an]=tan-1(a2-a1)[1+a1a2]+tan-1(a3-a2)[1+a2a3]+.+tan-1(an-an-1)[1+an-1an]=tan-1a2-tan-1a1+tan-1a3-tan-1a2+....tan-1an-tan-1an-1=tan-1an-tan-1a1=tan-1(an-a1)[1+a1an]=tan-1(an-an-1)[1+an-1an]=tan-15(n-1)[1+an-1an]=tan-1(5n-5)[1+an-1an]

Hence, the correct option is (C).


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