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Questions  

If tan3AtanA=k, then sin3AsinA is equal to

a
2kk-1,k∈R
b
2kk-1,k∈[1/3,3]
c
2kk-1,k∉[1/3,3]
d
k-12k,k∉[1/3,3]

detailed solution

Correct option is C

We have, tan3AtanA=k⇒3-tan2A1-3tan2A=k⇒tan2A=k-33k-1Now, sin⁡3Asin⁡A=3−4sin2⁡A=3−41+cot2⁡A=3−41+3k−1k−3=2kk−1Again, sin⁡3Asin⁡A=3−4sin2A⇒ 2kk−1=3−4sin2⁡A⇒ 4sin2⁡A=3−2kk−1⇒ sin2⁡A=k−34(k−1)⇒ 0≤k−34(k−1)≤1                    ∵0≤sin2⁡A≤1⇒ k<13 or k>3Hence, sin⁡3Asin⁡A=2kk−1, where k<13 or, k>3.

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