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Questions  

Let f(θ)=cotθ1tanθ+tanθ1cotθ,π<θ<3π4 , then f9π8 is equal to 

a
1+2
b
1+22
c
2−1
d
22−1

detailed solution

Correct option is B

f(θ)=1tan⁡θ1(1−tan⁡θ)+tan⁡θ⋅tan⁡θtan⁡θ−1=1−tan3⁡θtan⁡θ(1−tan⁡θ)=1+tan⁡θ+tan2⁡θtan⁡θ=1+2sin⁡2θ∴f9π8=1+2sin⁡9π4=1+22

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