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Q.If y=cos⁡−1(1−x21+x2)y = \cos^{-1}\left(\frac{1-x^2}{1+x^2}\right), 0<x<10 < x < 1, then dydx\frac{dy}{dx} is equal to

(a) 11+x2\frac{1}{1+x^2}
(b) 24+x\frac{2}{4+x}
(c) 21+x2\frac{2}{1+x^2}
(d) 2x+x2\frac{2}{x+x^2}
Rajasthan RbseRajasthan Board Senior Secondary Examination 2026MCQ· 1mImportance★★★★★
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Put x=tan⁡ϕx=\tan\phi so the expression simplifies to cos⁡−1(cos⁡2ϕ)=2ϕ=2tan⁡−1x\cos^{-1}(\cos2\phi)=2\phi=2\tan^{-1}x, whose derivative is standard.

Let x=tan⁡ϕx=\tan\phi. Then 1−x21+x2=1−tan⁡2ϕ1+tan⁡2ϕ=cos⁡2ϕ\dfrac{1-x^2}{1+x^2} = \dfrac{1-\tan^2\phi}{1+\tan^2\phi} = \cos2\phi.

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