Prove that tan−1[1+x+1−x1+x−1−x]=4π−21cos−1x, for −21≤x≤1.Answer: Proved.SolutionPut x=cos2θ, so 2θ=cos−1x and θ=21cos−1x. Then 1+x=2cosθ and 1−x=2sinθ (both non-negative on the stated range).1+x+1−x1+x−1−x=2cosθ+2sinθ2cosθ−2sinθ=cosθ+sinθcosθ−sinθ=1+tanθ1−tanθ=tan(4π−θ)Hencetan−1[1+x+1−x1+x−1−x]=4π−θ=4π−21cos−1x4π−21cos−1xCBSE Class 12 Board · 2011 · Subjective