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Questions  

If 1x0, then cos12x21 equals

a
2cos−1⁡x
b
π−2cos−1⁡x
c
2π−2cos−1⁡x
d
−2cos−1⁡x

detailed solution

Correct option is C

Let cos−1⁡x=θ.  Then, x=cos⁡θ.Also,       −1≤x≤0⇒−1≤cos⁡θ≤0⇒π2≤θ≤πNow,    cos−1⁡2x2−1=cos−1⁡(cos⁡2θ)=cos−1⁡(2π−2θ)⇒ cos−1⁡2x2−1=2π−2θ∵π2≤θ≤π⇒π≤2θ≤2π⇒0≤2π−2θ≤π⇒ cos−1⁡2x2−1=2π−2cos−1⁡x

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