Prove that sin−1(2x1−x2)=2cos−1x, 21≤x≤1.Answer: Proved: sin−1(2x1−x2)=2cos−1x.SolutionPut x=cosθ, so θ=cos−1x. For 21≤x≤1, θ∈[0,4π], hence 2θ∈[0,2π].Then 1−x2=1−cos2θ=sinθ (non-negative on [0,4π]), and2x1−x2=2cosθsinθ=sin2θSince 2θ∈[0,2π]⊂[−2π,2π],sin−1(2x1−x2)=sin−1(sin2θ)=2θ=2cos−1xsin−1(2x1−x2)=2cos−1xCBSE Class 12 Board · 2020 · Subjective