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Questions  

If x<0, then tan-11x equals

a
cot−1⁡x
b
-cot−1⁡x
c
−π+cot−1⁡x
d
−π-cot−1⁡x

detailed solution

Correct option is C

Let cot−1⁡x=θ. Then, x=cot⁡θAlso, x<0⇒cot⁡θ<0⇒π2<θ<πNow,          tan−1⁡1x⇒ tan−1⁡1x=tan−1⁡(tan⁡θ)⇒ tan−1⁡1x=tan−1⁡(−tan⁡(π−θ))  ⇒ tan−1⁡1x=tan−1⁡(tan⁡(θ−π))⇒ tan−1⁡1x=θ−π π2<θ<π⇒−π2<θ−π<0⇒ tan−1⁡1x=cot−1⁡x−π.

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