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Question

For a positive integer n, let fn(θ)=(tanθ2)(1+secθ)(1+sec2θ)(1+sec4θ)....(1+sec2nθ). Then

A
f2(π16)=1
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B
f3(π32)=1
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C
f4(π64)=1
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D
f5(π128)=1
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Solution

The correct options are
A f2(π16)=1
B f5(π128)=1
C f4(π64)=1
D f3(π32)=1

convert the given equation in the form of \sin and \cos then

we have

fn(θ)=sinθ2(1+cosθ)(1+cos2θ)(1+cos4θ).....(1+cos2nθ)cosθ2cosθcos2θcos4θ.....cos2nθ


=sinθ2(2cos2θ2)(2cos2θ)(2cos22θ).....(2cos2n1θ)cosθ2cosθcos2θcos4.....cos2nθ


=2n+1sinθ2cosθ2cosθcos2θ......cos2n1θcos2nθ


=sin2nθcos2nθ=tan2nθ

putting values


f2(π16)=tan22(π16)=1
f3(π32)=1,f4(π64)=1,f5(π128)=1


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